Search PubMed⌕ Search

Biomedical subjects

G Ramakrishnan

Publications and source records attributed to G Ramakrishnan.

29 records · Page 2Linked to original sources
PubMed1989

Genetic switching in the flagellar gene hierarchy of Caulobacter requires negative as well as positive regulation of transcription.

Caulobacter crescentus flagellar (fla, flb, or flg) genes are periodically expressed in the cell cycle and they are organized in a regulatory hierarchy. We have analyzed the genetic interactions required for fla gene expression by determining the effect of mutations in 30 known fla genes on transcription from four operons in the hook gene cluster. These results show that the flaO (transcription unit III) and flbF (transcription unit IV) operons are located at or near the top of the hierarchy. They also reveal an extensive network of negative transcriptional controls that are superimposed on the positive regulatory cascade described previously. The strong negative autoregulation observed for the flaN (transcription unit I), flbG (transcription unit II), and flaO (transcription unit III) promoters provides one possible mechanism for turning off fla gene expression at the end of the respective synthetic periods. We suggest that these positive and negative transcriptional interactions are components of genetic switches that determine the sequence in which fla genes are turned on and off in the C. crescentus cell cycle.

Bacteria↗
PubMed1989

Escherichia coli sigma 54 RNA polymerase recognizes Caulobacter crescentus flbG and flaN flagellar gene promoters in vitro.

A set of the periodically regulated flagellar (fla) genes of Caulobacter crescentus contain conserved promoter sequence elements at -24 and -12 that are very similar to the sequence of the nitrogen assimilation (Ntr) and nitrogen fixation (Nif) promoters of enteric bacteria and Rhizobium spp. Transcription from Ntr and Nif promoters requires RNA polymerase containing sigma 54 instead of the usual sigma 70 and, in the case of the Ntr promoters, is activated by the transcription factors NRI and NRII. We have now demonstrated that the C. crescentus flbG and flaN promoters, which contain the Ntr/Nif type of consensus sequence, are utilized by purified Escherichia coli sigma 54 RNA polymerase (E sigma 54) in the presence of NRI and NRII but not by the purified sigma 70 RNA polymerase (E sigma 70) of E. coli. Oligonucleotide-generated flbG promoter deletions that removed the highly conserved GG dinucleotide at -24 or the GC dinucleotide at -12 or altered the spacing between the -24 and -12 sequence elements prevented utilization of the flbG promoter by the E. coli E sigma 54. Transversions of T to G at positions -26 and -15 also inactivated flbG promoter function in the E. coli cell-free transcription system, while a transition of G to A at position -16 in the nonconserved spacer region had no effect. The C. crescentus flaO and flbF promoters, which do not contain the Ntr/Nif-type promoter consensus sequence, were not utilized by either purified E sigma 54 or E sigma 70 from E. coli. Our results help to define the features of the Ntr/Nif-type consensus sequence required for promoter utilization by purified E. coli E sigma 54 and support the idea that C. crescentus may contain a specialized polymerase with similar promoter specificity required for expression of a set of fla genes.

Bacteria↗
PubMed1988

Characterisation of blast cells during blastic phase of chronic myeloid leukaemia by immunophenotyping--experience in 60 patients.

The blast cell population of 60 patients with chronic myeloid leukaemia in blast crisis (CML-BC) were analyzed with a panel of monoclonal antibodies to determine the cell surface antigen phenotypes. In addition, cytochemical stains periodic acid Schiff (PAS), myeloperoxidase (MP), Sudan black B (SBB) and terminal deoxynucleotidyl transferase (TdT) were also utilized for subtyping. Nineteen cases (31.6%) expressed lymphoid phenotypes characteristic of common ALL cells and one case with extramedullary lymph node crisis expressed T-cell surface phenotypes. Thirty cases (50%) expressed solely myelomonocytic surface antigens with significant TdT activity in three. Cytochemical stains contributed to recognize only 57% of these myeloid blasts. Seven cases (11.7%) were with a mixture of heterogenous group of cells expressing phenotypic characteristics for various haemopoietic cells of different lineage--five of them from the cells of non-lymphoid series (myelomono-erythromegakaryocytic series) and the other two with cells from both lymphoid and myeloid series. Additionally, in two cases (3.3%), the precursor cells reacted only with the erythroid monoclonals. Finally, in one case, the blast cells remained unclassified due to nonreactivity with any of the monoclonals used but expressed significant TdT positivity. The response to uniform vincristine and prednisolone (V + P) therapy has shown that lymphoid blast crisis cases were highly responsive in contrast to the cases with non-lymphoid blast crisis (complete remission rate 86 vs 21.4%). The results confirm the evidence of multilineage blast crisis involving either single or mixed haemopoietic differentiation pathway and the utility of having phenotypic characterisation for designing protocols for chemotherapy in the CML patients at the time of blast crisis.

Adolescent↗
PubMed1988

Regulation of ompC and ompF expression in Escherichia coli in the absence of envZ.

The expression of the genes encoding the major outer membrane porin proteins OmpF and OmpC in Escherichia coli is regulated by ompR, which encodes the transcriptional activator protein OmpR, and envZ, which encodes a receptorlike protein located in the inner membrane. To examine the role of EnvZ in the expression of the osmoregulated porin genes, we analyzed the production of OmpF and OmpC in cells that lack envZ function. We show that EnvZ is required for the maximal production of OmpC in cells grown in minimal medium but is not essential for the efficient induction of OmpC that occurs during a shift to a high-osmolarity medium. In contrast, the production of OmpF in cells that lack envZ function was similar to that of the parent strain, whereas OmpF repression during a shift to a high-osmolarity medium was incomplete in the absence of EnvZ. These results are discussed in the context of the putative role of EnvZ in the expression of ompF and ompC.

Bacterial Outer Membrane Proteins↗
PubMed1986

Interaction of a transcriptional activator, OmpR, with reciprocally osmoregulated genes, ompF and ompC, of Escherichia coli.

The ompB locus, comprised of the genes ompR and envZ, regulates the expression of the genes ompF and ompC that encode the major porin proteins in the outer membrane of Escherichia coli K-12. OmpR is believed to activate transcription of ompF and ompC in a reciprocal manner depending upon the osmolarity of the culture medium. We were able to purify OmpR to homogeneity and to characterize its interaction with the porin gene promoters. The purified OmpR was shown to bind specifically to promoter fragments of both ompF and ompC. Deoxyribonuclease I footprinting was carried out in order to determine binding sites of OmpR in the promoter regions of the ompF and ompC. OmpR was found to protect the regions -105 to -60 of ompF and -102 to -78 of ompC, respectively. Two consensus sequences were found in these protected sites and are likely to play an important role in OmpR recognition of these promoter regions. The purified OmpR was also shown to be functionally active in transcription in vitro; OmpR activates the expression of ompF and inhibits the transcription of ompC from the most distal of its three tandem promoters, P3.

Bacterial Outer Membrane Proteins↗
PubMed1986

Regulation of the ompC gene of Escherichia coli. Involvement of three tandem promoters.

ompC expression in Escherichia coli K-12 is known to be regulated by the ompB locus, comprising the ompR and envZ genes, and the OmpR protein is believed to act as a positive transcriptional factor. We examined the transcriptional capability of the ompC gene in vitro and found that RNA polymerase could transcribe ompC without a requirement for other transcriptional factors. Furthermore, transcripts from three tandem promoters in ompC were identified in vitro. We employed oligonucleotide-directed site-specific mutagenesis to dissect the promoter region of the gene and assayed the promoters separately for transcriptional ability using fusions to the lacZ gene. The levels of beta-galactosidase indicate that ompC expression in vivo is dependent on the function of at least one of the upstream promoters. The function of OmpR appears to be the enhancement of a basal level of ompC expression. From the results of our experiments, the site of action of OmpR was deduced to be in the vicinity of the upstream promoters of ompC.

Base Sequence↗
PubMed1985

Anterior chemotherapy in esophageal cancer.

Front loading chemotherapy using methotrexate (200 mg/m2) alone or methotrexate (200 mg/m2) with cisplatin (20 mg/m2 daily for 5 days) was used in epidermoid carcinoma of esophagus. Evaluation after two courses showed objective response of 50% or greater in 48% of patients with methotrexate alone. Response rate was increased to 76.2% with addition of cisplatin to methotrexate. Small lesions (less than 10 cm) showed better response as compared to advanced cases. Therapy was generally well tolerated and good palliation was obtained even after the first course. Postchemotherpy treatment either with surgery or radiotherapy was tolerated without any major complications. The data confirm the short-term usefulness of initial chemotherapy with methotrexate and cisplatin in esophageal cancer. Results of prolonged follow-up will help to evaluate the role of front loading chemotherapy on long-term survival.

Adult↗
PubMed1985

Uncoupling of osmoregulation of the Escherichia coli K-12 ompF gene from ompB-dependent transcription.

The expression of the genes for the Escherichia coli K-12 outer membrane proteins, ompF and ompC, is subject to osmoregulation and responds to changes in the osmolarity of the growth medium. The transcription of these genes is dependent on the products of the regulatory locus ompB (comprising the genes ompR and envZ). The native promoter of ompF was replaced with an inducible lpp promoter to eliminate this transcriptional dependence of ompF expression on ompB. As a result, it was possible for the OmpF protein to be produced in an ompB mutant strain that does not normally express ompF. Surprisingly, the expression of ompF under the lpp promoter was still osmoregulated not only in the ompB+ strain but also in two ompB strains tested. These results indicate the involvement of a factor(s) besides the ompR and envZ gene products in the osmoregulation of OmpF production. This factor may interact with a sequence downstream of the ompF promoter. In addition, we show that the expression of ompF under the lpp promoter has no direct effect on ompC expression.

Bacterial Outer Membrane Proteins↗
PubMed1985

Pediatric Hodgkin's disease in India.

Twenty-one percent of all Hodgkin's disease in India was seen in the pediatric age groups at the Tata Memorial Hospital (Bombay, India). From 1975 to 1982, 151 cases of children were reviewed. The youngest presentation was at 3 years in three patients, with a marked male: female ratio of 5.5:1. Twenty-six patients were previously treated before referral while the remaining 125 cases were investigated and treated according to the prevalent protocols in 1975 to 1978 and 1979 to 1982. Clinical staging revealed 54% of patients in stages I and II with symptoms in 20%, and 46% of patients in stages III and IV with symptoms in 67%. Staging laparotomy was performed in 27 patients, with a total changes of staging in 17 children (63%). The mixed cell types (46%) and lymphocytic predominant types (31%) were the most common histologic presentations. Nine percent nodular sclerosis and 9% lymphocytic-depleted varieties were also observed. Five percent of all cases were not classifiable. Minimum adequate treatment was completed in 87 cases. Comparisons were made between the treatments administered to 40 patients during the initial period 1975 to 1978 when individualized treatment was administered, and the later 47 patients during the 1979 to 1982 period, when chemotherapy was the mainstay of treatment with involved field radiation.

Actuarial Analysis↗
PubMed1983

Total body irradiation in chronic myeloid leukemia.

Total body irradiation (TBI), given as 10 rad daily for five days a week for a total dose of 150 rad has been used in an attempt to control the chronic phase of chronic myeloid leukemia (CML). Thirteen patients with CML received fractionated TBI leading to rapid and good control of WBC count without any adverse reaction. The chronic phase of CML could also be controlled with TBI, even in three patients who were resistant to busulfan. Following TBI, WBC count remained under control for a period of 32 weeks as compared to 40 weeks following busulfan alone. Repeat TBI was also well tolerated with good response. It appears that TBI is an effective and safe therapy for controlling the chronic phase of CML.

Adolescent↗