Our Science – Adhya Website

Sankar Adhya, Ph.D.

Laboratory of Molecular Biology Head, Developmental Genetics Section Senior Investigator Building 37, Room 5138 National Cancer Institute 37 Convent Drive Bethesda, MD 20892 Phone:   301-496-2495 Fax:   301-480-7687 E-Mail:   sadhya@helix.nih.gov

Biography

Dr. Adhya received his Ph.D. from both the University of Calcutta and the University of Wisconsin. He was a research associate at the University of Rochester and Stanford University. He joined the Laboratory of Molecular Biology at the NCI in 1971. In 1994, Dr. Adhya was elected a member of the National Academy of Sciences and a Fellow of the Indian National Science Academy. He has also been an adjunct professor in the Department of Genetics at George Washington University since 1987.

Research

Regulation of Gene Transcription
Virtually every adaptation and developmental process originates at the level of gene regulation by transduction of extra- or intracellular signals. Transcription is the major target of regulation of gene expression. Our research interest covers modulation of transcription by DNA control elements and regulatory proteins, for example, repressors, activators, terminators, and antiterminators and their signal molecules. We have previously demonstrated transcriptional regulation both at the level of initiation by activators and repressors and at the level of elongation by terminators and antiterminators in the gal operon which encodes enzymes of D-galactose metabolism in E. coli. Presumably because of the amphibiotic nature of the biochemical pathway in carbon metabolism, the operon shows a multitude of controls even at the level of transcription initiation, and has been a paradigm system for studying control of transcription. The operon is transcribed from two promoters which are subject to both negative and positive control by at least four proteins, Gal repressor, Gal isorepressor, cyclic AMP receptor protein, and bacterial histone like protein, HU. The major highlight of our work in the past 2 years is determining the structure, assembly and dynamics of a higher order nucleoprotein complex containing DNA loop (repressosome) which regulates the gal promoters.

- Regulation by DNA looping. Synergistic repressor binding to two operators, O_E and O_I, encompassing the promoters P1 and P2, creates a DNA loop which inhibits transcription initiation from the gal promoters. A topologically closed loop, which is inflexible to torsional changes, disables the promoters by resisting DNA unwinding by RNA polymerase needed for open complex formation. In the repressosome, the DNA loop formation also requires piggybacking of the histone-like protein, HU, by GalR to an architecturally critical position on DNA which in turn further stabilizes an intrinsic GalR-GalR interaction. By structure-based genetic and biochemical analysis, we have defined the GalR-GalR the GalR-HU interaction domains. We have demonstrated the geometry of the DNA loop to be antiparallel by evaluation of the DNA elastic energy of the loop and by atomic force microscopy. Furthermore, by single molecule analysis, we determined the kinetic and thermodynamic parameters of the repressome stability.

- Regulation by DNA unlooping. Repressor binding to the upstream operator O_E alone, in the absence of DNA looping, represses one promoter P1 and activates the other P2. We have shown that both inhibition and stimulation of transcription requires the presence of specific regions of the alpha subunit of RNA polymerase. Our results suggest that Gal repressor inhibits or stimulates transcription initiation by disabling or stimulating RNA polymerase activity at a post-binding step by directly or indirectly altering the specific domain of RNA polymerase by a direct GalR-RNA polymerase contact(s) to an unfavorable or to a more favorable state, respectively. We have shown that base unpairing during isomerization of the closed to open complex at the P1 promoter is an asynchronous process with a rate-limiting step as observed by 2, AP fluorescence kinetics. We have also demonstrated that during the asynchronous base unpairing process, the adenine at the -11 position plays a 'master' role. Furthermore, CRP stimulates P1 transcription by enhancing, and GalR represses the same by quenching the rate-limiting step.

- Bacteriophage Therapy. Bacteriophage (phage) have been used in treatment of bacterial infections in humans since their discovery many decades ago, but the practice was discontinued because of lack of scrutiny required for Western medicine, and because of the discovery of potent antibiotics. Widespread occurrence today of drug resistant pathogens motivated us to attempt to revive phage based diagnosis and treatment of bacterial infections. We have demonstrated complete rescue of animals with experimental bacterimia caused by Escherichia coli, Salmonella typhimurium, and vanomycin resistant Enterococcus faecium by treatment with native or special mutants of cognate bacteriophages. We are also currently developing engineered phage to detect and treat infection by by yersinia pestis.

This page was last updated on 6/11/2008.