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PubMed1991

Repression of the human glycoprotein hormone alpha-subunit gene by glucocorticoids: evidence for receptor interactions with limiting transcriptional activators.

Expression of the glycoprotein hormone alpha gene is regulated divergently by glucocorticoids in different cell types. Coexpression of the glucocorticoid receptor (GR) with an alpha-CAT reporter gene caused activation of alpha promoter activity in fibroblasts, but repression in JEG-3 choriocarcinoma cells, indicating that cell-specific factors dictate positive vs. negative regulation of this promoter by GR. Cell-specific sequences and other enhancer elements in the the alpha gene have been relatively well characterized in JEG-3 cells, and this model was used to further examine the mechanism of transcriptional repression by glucocorticoids. Promoter mutagenesis indicated that the degree of GR-mediated repression was impaired by a variety of deletional and site-directed mutations between -171 and -111 bp, a region that includes both cell-specific and cAMP response elements (CREs). In an attempt to further localize a negative glucocorticoid response element (GRE) sequence, binding studies were used to assess GR interactions with alpha promoter DNA sequences. Using avidin-biotin complex DNA binding assays, a series of overlapping alpha promoter DNA sequences between -170 to 29 basepairs were tested, but each failed to bind GR, whereas a control GRE avidly bound receptor. Similarly, in competition assays in transfected CV-1 cells, the alpha gene 5'-flanking sequence did not compete for GR stimulation of a glucocorticoid responsive reporter gene, whereas a sequence that contains known GR-binding sites (murine mammary tumor virus) effectively inhibited GR-mediated expression. The absence of high affinity GR-binding sites in the alpha promoter suggested that mutations that affected GR inhibition may have eliminated recognition sites for transactivators, which are themselves targets for the GR, rather than altering specific negative GRE sites in the DNA sequence. To examine this possibility, GR repression was studied using chimeric transcription factors. The transcription-activating domains of several different proteins (CREB, thyroid hormone receptor, or VP16) were linked to the DNA-binding domain of Gal-4, and transcription was driven by the Gal-4 recognition site (UAS). GR markedly repressed transactivation by Gal-4-CREB and, to a lesser degree, the Gal-4-thyroid hormone receptor and Gal-4-VP16 chimeric proteins. Repression occurred when UAS was linked to either the alpha promoter or to the E1B promoter. Thus, inhibition occurs in the absence of either the CRE or the proximal alpha promoter. These results support a mechanism in which GR-mediated repression in JEG-3 cells occurs by receptor interference with the transactivating potential of enhancer-binding proteins or associated transcription factors.

Base Sequence↗
PubMed1997

Repression of cAMP-induced expression of the mouse P450 17 alpha-hydroxylase/C17-20 lyase gene (Cyp17) by androgens.

In primary cultures of mouse Leydig cells, testosterone represses the cAMP-induced de novo synthesis of P450 17 alpha-hydroxylase/C17-20 lyase (P450c17) protein and the accumulation of P450c17 mRNA, via an androgen receptor (AR)-mediated mechanism. To examine the mechanism by which androgens repress the cAMP-induced expression of the mouse Cyp17 gene, constructs containing 5'-flanking sequences of the mouse Cyp17 linked to the chloramphenicol acetyltransferase (CAT) reporter gene were cotransfected into MA-10 tumor Leydig cells with a mouse AR expression plasmid. In the presence of dihydrotestosterone, the cAMP-induced expression of a reporter construct containing -1021 bp of Cyp17 promoter sequences was repressed. In contrast, no repression by dihydrotestosterone was observed when the -1021 bp Cyp17-CAT construct was cotransfected with a human AR expression plasmid missing the second zinc finger of the DNA-binding domain, indicating that DNA binding is involved in AR-mediated repression of Cyp17 expression. Analysis of deletions -346 bp of 5'-flanking region of the mouse Cyp17 promoter are sufficient to confer androgen repression of the cAMP-induced expression of Cyp17. Deoxyribonuclease I footprinting analysis indicated that the AR interacts with sequences between -330. and -278 bp of the Cyp17 promoter. This region overlaps with the previously identified cAMP-responsive region located between -346 and -245 bp of the Cyp17 promoter. These results suggest that AR-mediated repression involves binding of the AR to sequences in the cAMP-responsive region of the Cyp17 promoter, possibly interfering with the binding of the protein(s) that mediate cAMP induction of Cyp17.

Animals↗
PubMed1998

A dual mechanism mediates repression of NF-kappaB activity by glucocorticoids.

Repression of nuclear factor (NF)-kappaB-dependent gene expression is one of the key characteristics by which glucocorticoids exert their antiinflammatory and immunosuppressive effects. In vitro studies have shown protein-protein interactions between NF-kappaB and the glucocorticoid receptor, possibly explaining their mutual repression of transcriptional activity. Furthermore, glucocorticoid-induced transcription of IkappaBalpha was presented as a mechanism in mediation of immunosuppression by glucocorticoids. At present, the relative contribution of each mechanism has not been investigated. We show that dexamethasone induced IkappaBalpha gene transcription in human pulmonary epithelial A549 cells. However, this enhanced IkappaBalpha synthesis did not cause repression of NF-kappaB DNA-binding activity. In addition, dexamethasone was still able to inhibit the expression of NF-kappaB target genes (cyclooxygenase-2, intercellular adhesion molecule-1) in the absence of protein synthesis. Furthermore, we show that the antihormone RU486 did not induce IkappaBalpha expression. However, RU486 was still able to induce, albeit less efficiently, both glucocorticoid- and progesterone receptor-mediated repression of endogenous NF-kappaB target gene expression in A549 cells and the breast cancer cell line T47D, respectively. Taken together, these results indicate that induced IkappaBalpha expression accounts for only part of the repression of NF-kappaB activity by glucocorticoids and progestins. In addition, protein-protein interactions between NF-kappaB and the glucocorticoid or progesterone receptor, resulting in repression of NF-kappaB activity, seem also to be involved. We therefore conclude that NF-kappaB activity is repressed via a dual mechanism involving both protein-protein interactions and induction of IkappaBalpha.

Animals↗
PubMed2004

Repression of Dpp targets in the Drosophila wing by Brinker.

Patterning along developing body axes is regulated by gradients of transcription factors, which activate or repress different genes above distinct thresholds. Understanding differential threshold responses requires knowledge of how these factors regulate transcription. In the Drosophila wing, expression of genes such as omb and sal along the anteroposterior axis is restricted by lateral-to-medial gradients of the transcriptional repressor Brinker (Brk). omb is less sensitive to repression by Brk than sal and is consequently expressed more laterally. Contrary to previous suggestions, we show that Brk cannot repress simply by competing with activators, but requires specific repression domains along with its DNA-binding domain. Brk possesses at least three repression domains, but these are not equivalent; one, 3R, is sufficient to repress omb but not sal. Thus, although sal and omb show quantitative differences in their response to Brk, there are qualitative differences in the mechanisms that Brk uses to repress them.

Amino Acid Sequence↗
PubMed2005

Analyses of SUM1-1-mediated long-range repression.

In Saccharomyces cerevisiae, local repression is promoter specific and localized to a small region on the DNA, while silencing is promoter nonspecific, encompasses large domains of chromatin, and is stably inherited for multiple generations. Sum1p is a local repressor protein that mediates repression of meiosis-specific genes in mitotic cells while the Sir proteins are long-range repressors that stably silence genes at HML, HMR, and telomeres. The SUM1-1 mutation is a dominant neomorphic mutation that enables the mutant protein to be recruited to the HMR locus and repress genes, even in the absence of the Sir proteins. In this study we show that the mutation in Sum1-1p enabled it to spread, and the native HMR barrier blocked it from spreading. Thus, like the Sir proteins, Sum1-1p was a long-range repressor, but unlike the Sir proteins, Sum1-1p-mediated repression was more promoter specific, repressing certain genes better than others. Furthermore, repression mediated by Sum1-1p was not stably maintained or inherited and we therefore propose that Sum1-1p-mediated long-range repression is related but distinct from silencing.

Binding Sites↗
PubMed2001

Nuclear receptor-mediated repression of human cholesterol 7alpha-hydroxylase gene transcription by bile acids.

Hydrophobic bile acids strongly repressed transcription of the human cholesterol 7alpha-hydroxylase gene (CYP7A1) in the bile acid biosynthetic pathway in the liver. Farnesoid X receptor (FXR) repressed CYP7A1/Luc reporter activity in a transfection assay in human liver-derived HepG2 cells, but not in human embryonic kidney (HEK) 293 cells. FXR-binding activity was required for bile acid repression of CYP7A1 transcription despite the fact that FXR did not bind to the CYP7A1 promoter. FXR-induced liver-specific factors must be required for mediating bile acid repression. Bile acids and FXR repressed endogenous CYP7A1 but stimulated alpha-fetoprotein transcription factor (FTF) and small heterodimer partner (SHP) mRNA expression in HepG2 cells. Feeding of rats with chenodeoxycholic acid repressed CYP7A1, induced FTF, but had no effect on SHP mRNA expression in the liver. FTF strongly repressed CYP7A1 transcription in a dose-dependent manner, and SHP further inhibited CYP7A1 in HepG2 cells, but not in HEK 293 cells. FXR only moderately stimulated SHP transcription, whereas FTF strongly inhibited SHP transcription in HepG2 cells. Results revealed that FTF was a dominant negative factor that was induced by bile acid-activated FXR to inhibit both CYP7A1 and SHP transcription. Differential regulation of FTF and SHP expression by bile acids may explain the wide variation in CYP7A1 expression and the rate of bile acid synthesis and regulation in different species.

Animals↗
PubMed2002

Carbon catabolite repression by the catabolite control protein CcpA in Staphylococcus xylosus.

Carbon catabolic repression (CR) by the catabolite control protein CcpA has been analyzed in Staphylococcus xylosus. Genes encoding components needed to utilize lactose, sucrose, and maltose were found to be repressed by CcpA. In addition, the ccpA gene is under negative autogenous control. Among several tested sugars, glucose caused strongest CcpA-dependent repression. Glucose can enter S. xylosus in nonphosphorylated form via the glucose uptake protein GlcU. Internal glucose is then phosphorylated by the glucose kinase GlkA. Alternatively, glucose can be transported and concomitantly phosphorylated by glucose-specific permease(s) of the phosphotransferase system (PTS). S. xylosus mutant strains deficient in GlcU or GlkA showed partial relief of glucose-specific, CcpA-dependent repression. Likewise, blocking PTS activity completely by inactivation of the gene encoding the general PTS protein enzyme I resulted in diminished glucose-mediated repression. Thus, both glucose entry routes contribute to glucose-specific CR in S. xylosus. The sugar transport activity of the PTS is not required to trigger glucose-specific repression. The phosphocarrier protein HPr however, is absolutely essential for CcpA activity. Inactivation of the HPr gene led to a complete loss of CR. Repression is also abolished upon inactivation of the HPr kinase gene or by replacing serine at position 46 of HPr by alanine. These results clearly show that HPr kinase provides the signal, seryl-phosphorylated HPr, to activate CcpA in S. xylosus.

Bacterial Proteins↗
PubMed2003

ING1 represses transcription by direct DNA binding and through effects on p53.

The ING family of proteins is involved in the regulation of diverse processes ranging from cell cycle and cellular senescence to apoptosis. These effects are most likely through activation of acetylation-dependent pathways that ultimately alter gene expression. Despite reports linking ING to p53 activation, the molecular basis of how ING activates p53 function has not been elucidated. In this study, we found that a subset of ING family members strongly repressed human alpha-fetoprotein (AFP) promoter activity but stimulated the p21(WAF1) promoter in parallel experiments in the same cell type, similar to the effects of p53. The p47(ING1a) isoform also repressed AFP promoter activity, but in contrast to other ING isoforms, it repressed the p21(WAF1) promoter. p47(ING3) up-regulated p21(WAF1) promoter activity, but it did not have any effect on the AFP promoter. ING1b and ING2 also repressed the AFP promoter in Hep3B p53-null cell lines, and p53 coexpression enhanced this transcriptional repression. Suppression of AFP gene transcription by ING was strongly dependent on AT-motifs that bind to the hepatocyte nuclear factor 1 (HNF1) transcription factor. Indeed, electrophoretic mobility shift assays confirmed that HNF1 binds to AT-motifs, but we found, surprisingly, that the ING1 complexes binding to these AT-motifs were devoid of HNF1 protein. Both ING1 and p53 were able to suppress AFP transcription and cause p21 induction; hSIR2, a negative regulator of the p53 protein, showed the opposite effects on the AFP promoter and, like HDAC1, repressed p21 promoter activity. In addition, we found that p33(ING1b) physically interacts with hSIR2, reverses its ability to induce the AFP promoter, and induces acetylation of p53 residues at Lys(373) and/or Lys(382). These findings provide novel evidence that p33(ING1b) represses AFP transcription by at least two mechanisms, one of which includes p53. The first is by binding to the AT-motif and excluding HNF1 binding while possibly targeting HAT activity to promoter regions, and the second is by increasing the levels of active, acetylated p53 via binding and inhibiting the ability of hSIR2 to deacetylate p53 protein.

Acetylation↗
PubMed1992

Cell-specific glucocorticoid repression of calcitonin/calcitonin gene-related peptide transcription. Localization to an 18-base pair basal enhancer element.

We have investigated the mechanisms underlying cell-specific glucocorticoid repression of calcitonin/calcitonin gene-related peptide (CGRP) gene expression. Treatment with the synthetic glucocorticoid dexamethasone has been shown to decrease mRNA levels in the 44-2C thyroid C cell line. Nuclear run-on assays showed that dexamethasone repressed transcription 2-3-fold in 44-2C cells. In contrast, dexamethasone stimulated calcitonin/CGRP transcription 4-6-fold in the CA77 thyroid C cell line. Transient transfection assays were used to map repression of reporter gene activity in 44-2C cells to a neuroendocrine cell-specific enhancer located between -920 and -1125 base pairs (bp). Within this region, an 18-bp element was found that conferred both full basal enhancer activity and dexamethasone-dependent repression in 44-2C cells. The 18-bp region contains possible binding sites for AP-1 and helix-loop-helix transcription factors as well as a glucocorticoid receptor half-site. Colocalization of repression and enhancer activity was then investigated in other cell lines. In CA77 cells, while the 920-1125 region strongly enhanced transcription, the 18-bp region conferred only partial activation and dexamethasone had little effect on reporter gene activity. Dexamethasone did not repress the calcitonin/CGRP activity in the heterologous HeLa and Rat1 fibroblast cell lines. These results suggest that glucocorticoids repress transcription of the calcitonin/CGRP gene by inhibiting cell-specific transcription factor activity.

Animals↗
PubMed1996

TATA-dependent repression of human immunodeficiency virus type-1 transcription by the adenovirus E1A 243R oncoprotein.

Human adenovirus E1A oncoprotein activates or represses transcription from a variety of viral and cellular promoters by several complex mechanisms. The E1A products, 289R and 243R, have differential effects on transcription directed by the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR). Previous reports indicate that repression of HIV-1 LTR-directed gene expression by E1A 243R is mediated through the kappa B enhancer elements located between nucleotides -105 and -82 relative to the transcription initiation start site (+1). Results from this study suggest a novel mechanism for transcriptional repression of the HIV-1 LTR by E1A 243R that is enhancer-independent and that is mediated through basal HIV-1 promoter elements. Transient expression assays, in which 5'-truncated or site-directed mutant HIV-1 LTR-CAT reporters were tested for their response to repression mediated by wild-type or mutant 243R, demonstrate that LTR sequences upstream of -31 relative to the transcription initiation start site (+1) and inclusive of the enhancer elements are dispensable for 243R-mediated repression. The ability of 243R to repress HIV-1 basal promoter activity requires both an intact N-terminus of E1A 243R and the TATA element within the HIV-1 promoter. These results support a novel mechanism for E1A 243R-induced transcriptional repression that is enhancer-independent and that targets directly the general transcription machinery.

Adenovirus E1A Proteins↗
PubMed1996

Transcriptional repression by the proto-oncogene BCL-6.

In up to 45% of reported cases of the non-Hodgkin's lymphoma, diffuse large cell lymphoma, there are translocations of the BCL-6 gene, which are presumed to deregulate its expression. The BCL-6 protein, which is unmutated in these lymphomas, contains six Krüppel-like zinc fingers at its carboxy terminus and a 121 amino acid domain at its amino terminus, termed the POZ domain, which bears homology with amino terminal domains in a subset zinc finger transcription factors. In this study, we tested whether BCL-6 regulates transcription and if the POZ domain has a role in this function. The BCL-6 POZ domain, when fused to the GAL4 DNA binding domain, strongly repressed transcriptional activation initiated from several different promoters including the SV40 enhancer/promoter. Repression was also observed when the fusion protein was bound at a distance of 200 bp 5' of the promoter. When the GAL4/BCL6 POZ domain fusion protein was expressed in yeast, it was able to homodimerize in the nucleus. Nevertheless, in contrast with mammalian cells, the fusion protein did not repress transcription. To test the ability of the full length BC1-6 protein to repress transcription when bound to DNA through its zinc finger DNA binding domain, high affinity BCL-6 binding sites were selected from a pool of random oligonucleotides. Full length BCL-6 was able to strongly repress transcription when bound to its cognate site cloned upstream of the thymidine kinase promoter. This repression was mediated, in large measure, by the POZ domain, although a variant of BCL-6 lacking the POZ domain was able to repress transcription modestly. The ability of BCL-6 to function as a transcriptional repressor may contribute to its ability to transform B lymphocytes in diffuse large cell lymphoma.

Base Sequence↗
PubMed1996

The gap protein knirps mediates both quenching and direct repression in the Drosophila embryo.

Transcriptional repression is essential for establishing localized patterns of gene expression during Drosophila embryogenesis. Several mechanisms of repression have been proposed, including competition, quenching and direct repression of the transcription complex. Previous studies suggest that the knirps orphan receptor (kni) may repress transcription via competition, and exclude the binding of the bicoid (bcd) activator to an overlapping site in a target promoter. Here we present evidence that kni can quench, or locally inhibit, upstream activators within a heterologous enhancer in transgenic embryos. The range of kni repression is approximately 50-100 bp, so that neighboring enhancers in a modular promoter are free to interact with the transcription complex (enhancer autonomy). However, kni can also repress the transcription complex when bound in promoter-proximal regions. In this position, kni functions as a dominant repressor and blocks multiple enhancers in a modular promoter. Our studies suggest that short-range repression represents a flexible form of gene regulation, exhibiting enhancer- or promoter-specific effects depending on the location of repressor binding sites.

Amino Acid Sequence↗
PubMed2001

Long-range repression by multiple polycomb group (PcG) proteins targeted by fusion to a defined DNA-binding domain in Drosophila.

A tethering assay was developed to study the effects of Polycomb group (PcG) proteins on gene expression in vivo. This system employed the Su(Hw) DNA-binding domain (ZnF) to direct PcG proteins to transposons that carried the white and yellow reporter genes. These reporters constituted naive sensors of PcG effects, as bona fide PcG response elements (PREs) were absent from the constructs. To assess the effects of different genomic environments, reporter transposons integrated at nearly 40 chromosomal sites were analyzed. Three PcG fusion proteins, ZnF-PC, ZnF-SCM, and ZnF-ESC, were studied, since biochemical analyses place these PcG proteins in distinct complexes. Tethered ZnF-PcG proteins repressed white and yellow expression at the majority of sites tested, with each fusion protein displaying a characteristic degree of silencing. Repression by ZnF-PC was stronger than ZnF-SCM, which was stronger than ZnF-ESC, as judged by the percentage of insertion lines affected and the magnitude of the conferred repression. ZnF-PcG repression was more effective at centric and telomeric reporter insertion sites, as compared to euchromatic sites. ZnF-PcG proteins tethered as far as 3.0 kb away from the target promoter produced silencing, indicating that these effects were long range. Repression by ZnF-SCM required a protein interaction domain, the SPM domain, which suggests that this domain is not primarily used to direct SCM to chromosomal loci. This targeting system is useful for studying protein domains and mechanisms involved in PcG repression in vivo.

ATP-Binding Cassette Transporters↗
PubMed1994

The Aspergillus nidulans CREA protein mediates glucose repression of the ethanol regulon at various levels through competition with the ALCR-specific transactivator.

Carbon catabolite repression in Aspergillus nidulans is mediated by a negative-acting protein coded by the creA gene. We have investigated how CREA controls the expression of the ethanol regulon genes. CREA is a major component of the control of this regulon. Its presence in the cell results in a permanent, albeit partial, repression of the alc genes under all physiological growth conditions, even when the fungus is grown on carbon sources considered to be non-repressing. A crucial step in the control processes is the repression of the positive-acting specific regulatory gene alcR, by the binding of CREA on its cognate target sites on the alcR promoter. The removal of one of these targets, URSA, results in a 50% derepression of the alcR gene. Furthermore, the presence of this sequence contributes directly to the low alcR expression under nonrepressing conditions and reduces alcR promoter function by at least 100-fold. CREA acts both on the regulatory gene alcR and directly on the two structural genes alcA and aldA, as glucose repression of the latter genes occurs in strains where alcR transcription is driven by a strong constitutive and derepressed promoter. In vivo and in vitro competition experiments show that CREA acts by competing directly with the binding of the ALCR activator for the same region of the alcR promoter, a region which encompasses overlapping targets for both regulatory proteins. These data are consistent with a model in which the activating and repressing regulatory proteins compete to regulate expression of the ethanol regulon genes.

Alcohol Dehydrogenase↗
PubMed1977

Mutants of Saccharomyces cerevisiae resistant to carbon catabolite repression.

Mutants with defective carbon catabolite repression have been isolated in the yeast Saccharomyces cerevisiae using a selective procedure. This was based on the fact that invertase is a glucose repressible cell wall enzyme which slowly hydrolyses raffinose to yield fructose and that the inhibitory effects of 2-deoxyglucose can be counteracted by fructose. Repressed cells were plated on a raffinose--2-doexyglucose medium and the resistant cells growing up into colonies were tested for glucose non-repressible invertase and maltase. The yield of regulatory mutants was very high. All were equally derepressed for invertase and maltase, no mutants were obtained with only non-repressible invertase synthesis which was the selected function. A total of 61 mutants isolated in different strains were allele tested and could be attributed to three genes. They were all recessive. Mutants in one gene had reduced hexokinase activities, the other class, located in a centromere linked gene, had elevated hexokinase levels and was inhibited by maltose. Mutants in a third gene were isolated on a 2-deoxyglucose galactose medium and had normal hexokinase levels. A partial derepression was observed for malate dehydrogenase in all mutants. Isocitrate lyase, however, was still fully repressible.

Carbon↗
PubMed1978

Evidence for translational repression of arginine biosynthetic enzymes in Escherichia coli: altered regulation in a streptomycin-resistant mutant.

The formation and repressibility of the arginine biosyntietic enzymes acetylornithine delta-aminotransferase (EC 2.6.1.11), acetylornithine deacetylase (EC 3.5.1.16), ornithine carbamoyltransferase (EC 2.1.3.3), and argininosuccinate lyase (EC 4.3.2.1) were studied in an Escherichia coli W derivative (strain 250-10) that carries (a) a mutant allele of the argR regulatory gene causing a diminished repression-derepression range and (b) a streptomycin resistance mutation. In comparison with the streptomycin-sensitive parent 250, all four enzymes (a) are formed as smaller proportions of the total protein (overall range, 12% to 71%), whether the conditions are repressive (arginine excess) or derepressive (arginine restriction), and (b) show increased repressibility ratios, the carbamoyltransferase giving the largest increase (from 5.7 to 25.0). These effects appear to depend on the concurrent expression of the regulatory-gene and streptomycin resistance mutations, as indicated by analogous experiments with canavanine-resistant mutants of 250-10 that have partial argR- character. The results provide evidence for translational repression in the arginine system, and are interpreted in terms of a functional interaction of a mutant arginine repressor with a mutant S12 ribosomal protein. The locale of translational repression may be near the site of S12, and this mode of regulation may involve initiational selectivity of groupwise recognizable arginine messenger RNA's.

Acetamides↗
PubMed1975

Catabolite repression in Escherichia coli K12 mutants defective in glucose transport.

The phenomenon of glucose catabolite repression was studied in Escherichia coli mutants unable to transport this carbohydrate. The pts I,H mutant P34 was much less sensitive to permanent and transient repressive effect of glucose on beta-galactosidase synthesis than parental type. The 1103 mutant with lack of enzyme 1 of the phosphoenolpyruvate-dependent phosphotransferase system (ptsI) behaves as well as P34 mutant after addition of glucose to casamino acids mineral medium. But in minimal medium with succinate as the sole source of carbon cells of the 1103 mutant (in accordance with the data of Perlman and Pastan, 1969) show hightened sensibility to transient glucose repression. The effect of hypersensibility disappears when the lacI mutation rendering the beta-galactosidase synthesis to costitutivity is introduced in 1103 mutant. It is shown that the hightened sensibility of beta-galactosidase synthesis to glucose transient repression in 1103 mutant is not an effect of the pts mutation and most probably is due to "inducer exclusion" of the lac operon. It is also shown that if one introduces the P34 mutation in strain devoided of one of the enzymes II for glucose (gptA) (and due to this resistant to glucose catabolite repression) then the level of resistance in double mutant does not increase in spite of considerable supression of 14C glucose accumulation. It is discussed the role of separate components of Escherichia coli K12 glucose transport system in realization of the phenomenon of catabolite repression.

Biological Transport↗
PubMed1987

Separate regulatory systems for the repression of metE and btuB by vitamin B12 in Escherichia coli.

Synthesis of the btuB-encoded outer membrane receptor for vitamin B12 and the metE-encoded homocysteine methyltransferase is repressed by growth of Escherichia coli in the presence of vitamin B12. The regulation by vitamin B12 of the production of beta-galactosidase in strains carrying btuB-lac or metE-lac operon fusions indicated that repression of both genes operates at the transcriptional level. Selection for expression of these fusions under repressive conditions allowed isolation of second-site mutations in which repressibility by vitamin B12 had been lost. Mutations in metH and metF prevented vitamin B12-dependent regulation of metE, but not that of btuB. Mutations in btuB and other genes involved in uptake of the vitamin eliminated or reduced repression. Mutations in the newly identified gene, btuR, controlled the repressibility of btuB, but had no effect on metE regulation. The btuR gene resides at 27.9 min on the genetic map in the gene order cysB-topA-btuR-trp; it acts in a trans-dominant manner and appears to encode a repressor of btuB transcription.

Bacterial Outer Membrane Proteins↗