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PubMed1996

Transcription of the AAC1 gene encoding an isoform of mitochondrial ADP/ATP carrier in Saccharomyces cerevisiae is regulated by oxygen in a heme-independent manner.

The mitochondrial ADP/ATP carrier in Saccharomyces cerevisiae is encoded by three independent genes. AAC1, AAC2, and AAC3. In this work, we analysed the 5' upstream region of AAC1 by sequencing and by mapping the transcription initiation site of the gene. By monitoring the level of AAC1 mRNA and the beta-galactosidase activity of AAC1-lacZ fusion constructs, we showed that expression of AAC1 is subjected to regulation by oxygen. In contrast to the other two AAC genes, the effect of oxygen on AAC1 is not mediated by heme and heme-dependent transcription factors. The AAC1 expression was reduced eightfold in anaerobically grown cells compared to expression in cells grown aerobically, but it was not affected by the nature of carbon source used for growth. The data presented show that AAC1 expression, while constitutive under all aerobic conditions tested, is repressed during anaerobiosis in a heme-independent manner.

Aerobiosis↗
PubMed1968

N-acetylglucosamine assimilation in Escherichia coli and its relation to catabolite repression.

The ability of N-acetylglucosamine to enhance catabolite repression by glucose was studied by using cultures grown on a combination of these substrates. Under these conditions, it was shown that two-thirds of the N-acetylglucosamine utilized was routed into dissimilatory pathways, whereas the remaining one-third was channeled into biosynthesis. It was established that over 50% of the N-acetylglucosamine assimilated was incorporated directly into amino sugar polymers. It was also shown that this exogenous supply of N-acetylglucosamine was in fact used preferentially over glucose as the precursor for amino sugar polymer biosynthesis. These findings provided support for the prediction that catabolite repression in Escherichia coli may be interrelated with certain reactions involved in amino sugar biosynthesis.

Carbon Dioxide↗
PubMed1992

Amylase cis-acting sequences mediate the alleviation of glucose repression by cAMP in Drosophila.

alpha-Amylase gene expression is highly repressed by dietary glucose in Drosophila melanogaster. This glucose effect can be alleviated by exogenous adenosine 3':5'-cyclic monophosphate (cAMP). Here, we show that the relief of glucose repression by cAMP occurs at the level of amylase mRNA abundance. Furthermore, exogenous cAMP was shown to alleviate glucose repression of the transient expression of an amylase gene construct in transformed Amynull larvae. This construct contains only 109 base pairs of the promoter region; this is the minimal length of upstream sequence which is necessary for wild-type levels of amylase gene expression. These results indicate that cis-acting promoter elements located close to the transcriptional start site of the Drosophila amylase gene mediate both glucose repression and the cAMP-derepression effects.

Animals↗
PubMed1973

Aspartokinase of Myxococcus xanthus: "feedback stimulation" by required amino acids.

The aspartokinase activity found in extracts of the bacterium Myxococcus xanthus was subject to feedback inhibition and feedback repression by l-threonine and l-lysine. Both types of inhibition were essentially additive. The required amino acids, l-isoleucine and l-methionine, caused considerable increase in the activity of the enzyme. This phenomenon is referred to as "feedback stimulation." The polyamine, spermidine, exerted strong enhancement of the activity even at 0.1 mM. Meso-diaminopimelate, although not inhibitory by itself, abolished the activation exerted by either l-isoleucine or l-methionine. The possible physiological significance of interactions between the various effectors is discussed.

Aspartic Acid↗
PubMed1984

Positive control of expression of the argECBH gene cluster in vitro by guanosine 5'-diphosphate 3'-diphosphate.

By using a cell-free system derived from Escherichia coli, it was found that guanosine 5'-diphosphate 3'-diphosphate (ppGpp) was a positive effector for expression of both wings of bidirectionally transcribed argECBH gene cluster. A 7- to 20-fold increase in the synthesis of both argininosuccinase (the argH enzyme) and N-acetylornithinase (the argE enzyme) resulted with added ppGpp (0.2 mM optimum). Synthesis of hybridizable argECBH mRNA was enhanced only 30 to 100% by added ppGpp. Of the various guanosine nucleotides tested, only pppGpp mimicked ppGpp. Added ppGpp had no important effect upon (i) measurable argE or argH enzyme activity, (ii) total protein synthesis in the cell-free system, or (iii) the rate of decay of hybridizable argECBH mRNA. With extracts of an argR+ strain, added ppGpp had no effect on the repression of enzyme or mRNA synthesis by L-arginine. By using a two-stage system in which the bulk of argECBH mRNA was synthesized while protein synthesis was delayed, we showed that ppGpp acted at some point during transcription.

Escherichia coli↗
PubMed1978

Hormonal regulation and the effects of glucose on tyrosine aminotransferase activity in adult rat hepatocytes cultured on floating collagen membranes.

Adult rat parenchymal hepatocytes can be maintained in primary culture on floating collagen membranes of prolonged periods of time. In this system the enzyme tyrosine aminotransferase is induced by glucagon, (10(-6) to 10(-8) M) hydrocortisone (10(-5) to 10(-8) M), and cyclic adenosine 3':5'-monophosphate (cAMP) (10(-4) to 10(-5) M). Epinephrine (10(-4) M) induces the enzyme only in the presence of hydrocortisone. Addition of actinomycin D inhibited the induction of tyrosine aminotransferase by hydrocortisone and cAMP. Maintenance of the cultured hepatocytes in the presence of glucose (3g/liter) results in partial suppression of the inducing effects of glucagon and cAMP. Cyclic quanosine 3':5'-monophosphate does not mimic the effects of glucose. These results demonstrate that the phenomenon of glucose repression of enzyme induction, demonstrated in vivo in mammalian liver, is independent of changes in levels of serum hormones, which occur in vivo as a result of glucose administration. This study also demonstrates that glucose repression is not mediated by changes in intracellular levels of cAMP and cyclic quanosine 3':5'-monophosphate.

Animals↗
PubMed1972

Regulation of the tyrosine biosynthetic enzymes in Salmonella typhimurium: analysis of the involvement of tyrosyl-transfer ribonucleic acid and tyrosyl-transfer ribonucleic acid synthetase.

Mutants of Salmonella typhimurium were isolated that require tyrosine for growth because of an altered tyrosyl-transfer ribonucleic acid (tRNA) synthetase. Extracts of one strain (JK10) contain a labile enzyme with decreased ability to transfer tyrosine to tRNA(Tyr) and a higher K(m) for tyrosine than the wild-type enzyme. Strain JK10 maintains repressed levels of the tyrosine biosynthetic enzymes when the growth rate is restricted due to limitation of charged tRNA(Tyr). Several second-site revertants of strain JK10 exhibit temperature-sensitive growth due to partially repaired, heat-labile tyrosyl-tRNA synthetase. The tyrosine biosynthetic enzymes are not derepressed in thermosensitive strains grown at the restrictive temperature. A class of tyrosine regulatory mutants, designated tyrR, contains normal levels of tyrosyl-tRNA synthetase and tRNA(Tyr). These results suggest that charging of tRNA(Tyr) is not necessary for repression. This conclusion is substantiated by the finding that 4-aminophenylalanine, a tyrosine analogue which causes repression of the tyrosine biosynthetic enzymes, is not attached to tRNA(Tyr) in vivo, nor does it inhibit the attachment reaction in vitro. A combined regulatory effect due to the simultaneous presence of tyrS and tyrR mutations in the same strain was detected. The possibility of direct participation of tyrosyl-tRNA synthetase in tyrosine regulation is discussed.

Aldehyde-Lyases↗
PubMed1993

Tat protein of human immunodeficiency virus type 1 represses expression of manganese superoxide dismutase in HeLa cells.

Using a HeLa cell line stably transfected with the tat gene from human immunodeficiency virus type 1, we have found that the expression of the regulatory Tat protein suppresses the expression of cellular Mn-containing superoxide dismutase (Mn-SOD). This enzyme is one of the cell's primary defenses against oxygen-derived free radicals and is vital for maintaining a healthy balance between oxidants and antioxidants. The parental HeLa cells expressed nearly equivalent amounts of Cu,Zn- and Mn-SOD isozymes. Those cells expressing the Tat protein, however, contained 52% less Mn-SOD activity than parental cells, whereas that of the Cu,Zn enzyme was essentially unchanged. The steady-state levels of Mn-SOD-specific RNAs were also lower in the HeLa-tat cell line than in the parental line. No difference was seen in the steady-state levels of Cu,Zn-SOD-specific RNAs. In addition to the decreased Mn-SOD-activity, HeLa-tat cell showed evidence of increased oxidative stress. Carbonyl proteins were markedly higher, and total cellular sulfhydryl content decreased in cell extracts at a faster rate, probably reflecting ongoing lipid peroxidation. HeLa and HeLa-tat extracts were incubated with radiolabeled Mn-SOD transcripts, and the reaction products were subjected to UV crosslinking, digestion with ribonuclease A, and electrophoretic analysis. The results suggest a direct interaction between Tat protein and Mn-SOD gene transcripts.

Blotting, Northern↗
PubMed1975

A model for diauxic growth.

A model is developed to predict batch and continuous culture behavior of fermentations on two different carbon and energy sources. The basic assumption of the model is that the permease for the favored substrate is constitutive, whereas the premease for the second substrate is subject to induction and catabolite repression. Simulations of the model show features of diauxic growth described in the literature.

Adenosine Triphosphate↗
PubMed1972

A possible ribosomal-directed regulatory system in Euglena gracilis. Carbon dioxide fixation.

It was shown that cycloheximide inhibits CO(2) fixation in Euglena cells in the dark, but no effect of chloramphenicol was found. The light-dependent CO(2) fixation was inhibited by chloramphenicol and by large amounts of cycloheximide, but was stimulated by small amounts of cycloheximide. The presence of the stimulatory concentration of cycloheximide abolished the inhibition effect of high concentrations of chloramphenicol. The results indicate that the light-dependent CO(2) fixation is controlled by a repression-derepression regulatory system, which seems to be independent of chlorophyll synthesis.

Carbon Dioxide↗
PubMed1977

Amylase production by thermomonospora curvata.

Thermomonospora curvata produces an extracellular inducible amylase which does not accumulate products repressive to cellulase production during growth on starch-cellulose ratios similar to those of compost.

Agriculture↗
PubMed1974

Regulation of nitrogenase synthesis by oxygen in Klebsiella pneumoniae.

Klebsiella pneumoniae does not fix N(2) under aerobic conditions. The two protein components required for nitrogenase activity were studied during aeration of cells in nitrogen-free media. Component II of nitrogenase was inactivated more slowly in vivo than component I during aeration. The rate of loss of component II was less than the rate of component II synthesis during derepression. No inactive components were detected in cells that had been growing on NH(4) (+) and then aerated in nitrogen-free medium. This supports the hypothesis that O(2) somehow represses the formation of nitrogenase.

Aerobiosis↗
PubMed1988

Lack of carbon substrate repression of uptake hydrogenase activity in Bradyrhizobium japonicum SR.

The expression of ex planta uptake hydrogenase (Hup) activity in Bradyrhizobium japonicum SR induced in the absence or presence of carbon substrates was compared. Hup activity was influenced by pH, indicating that acidification of induction medium with low buffering capacity resulting from carbon substrate metabolism inhibited Hup activity. Cell suspensions in medium with adequate buffering capacity and carbon substrate were limited in O2; increasing O2 availability to cells during induction stimulated Hup expression. The data showed a lack of carbon substrate repression of Hup activity in cell suspensions provided with adequate O2 and buffering capacity.

Carbohydrate Metabolism↗