Charles Michael Cashel, Ph.D.,M.D.
Section on Molecular Regulation
Global Regulation of Gene Expression by ppGpp
The long-term focus of our laboratory has been on uncovering the molecular-biological roles of two naturally occurring nucleotides termed alarmones, commonly abbreviated as (p)ppGpp that are present in bacteria and chloroplasts. These are analogs of GTP and GDP with ribosyl 3' hydroxyls blocked by pyrophosphate residues. Virtually all sources of nutritional, physical and metabolic stress result in an increase in cellular levels of ppGpp with seconds reaching levels near GTP within minutes. Reversal of stress can lead to disappearance with half-lives of less than a minute. Even when ppGpp levels rise in response to either stress or a result of stress-free artificial genetic manipulation, a large number of complex global regulatory effects on gene expression ensue at the transcriptional and metabolic levels. In Escherichia coli, regulatory effects are gene-specific, but with both inhibitory and stimulatory components. Overall, the transcriptional activities of about one-third of the genomic repertoire are modulated. Extensive physiological modulation occurs through interconnecting regulatory circuits. Gram negative bacteria employ ppGpp differently; its effects are largely indirect consequences of lowering GTP levels. Our current interests center on the roles of low basal level fluctuations of (p)ppGpp that not only determine rates of balanced exponential growth but also the growth rate dependence of cellular macromolecular components, DNA, RNA and protein. Currently our interests have expanded to also include exciting parallel studies with pppApp and ppApp, new analogs of ATP and of ADP. These have been discovered to bind to a probably widely conserved RNA polymerase at a site near the catalytic center of RNA polymerase distinct from the ppGpp bindng site.The source of pppApp is the same large enzyme that is responsible for (p)ppGpp synthesis in one instance and trace amounts are detected in E. coli.
Dr. Michael Cashel attended Amherst College (BA), then CRU School of Medicine (MD), then became a USPHS fellow at NIH working on sporulation and DNA crosslinking. A two-year USPHS genetics training grant led to discovery of a 32P-labeled spot on thin layer chromatograms formed during the operation of E.coli stringent response to amino acid starvation and a Ph.D. Dr. Cashel returned to NIH in 1967 to identify the chemical composition of the spot compounds contained analogs of GDP and GTP with a pyrophosphorylated ribosyl 3' hydroxyl [called (p)ppGpp]. His subsequent NIH career has involved studies of (p)ppGpp. This led to attempting to understand how global regulation of bacterial; gene expression by (p)ppGpp occurs to ensure survival of nutritional and physical stress. This in turn, led to studies of RNA polymerase and factors that interact with the transcription process to adjust physiological adaptations to a variety of stress conditions. Parallel phenomena are widespread in both bacteria & plants and in each kingdom the ppGpp responses to stress have many similarities. Since host organisms battle pathogenic bacteria through the use of different stresses, ppGpp responses has almost invariably been associated with mechanisms of pathogenesis and resistance to host defenses. Currently, discoveries of additional (p)ppGpp-like nucleotide analogs as well as cyclic and di-cyclic noncanonical regulatory nucleotides are emerging along with along with details of ppGpp involvement with transcription regulation, physiological and metabolic effects, control of growth rate, regulation of ribosome content, initiation of chromosomal DNA synthesis and transcription coupled repair of DNA lesions .
Potrykus K, Murphy H, Philippe N, Cashel M. ppGpp is the major source of growth rate control in E. coli. Environ Microbiol. 2011;13(3):563-575.
Mechold U, Potrykus K, Murphy H, Murakami KS, Cashel M. Differential regulation by ppGpp versus pppGpp in Escherichia coli. Nucleic Acids Res. 2013;41(12):6175-89.
Hogg T, Mechold U, Malke H, Cashel M, Hilgenfeld R. Conformational antagonism between opposing active sites in a bifunctional RelA/SpoT homolog modulates (p)ppGpp metabolism during the stringent response [corrected]. Cell. 2004;117(1):57-68.
Sobala M, Bruhn-Olszewska B, Cashel M, Potrykus K. Methylobacterium extorquens RSH Enzyme Synthesizes (p)ppGpp and pppApp in vitro and in vivo, and Leads to Discovery of pppApp Synthesis in Escherichia coli. Front Microbiol. 2019;10:859.
Fernández-Coll L, Maciag-Dorszynska M, Tailor K, Vadia S, Levin PA, Szalewska-Palasz A, Cashel M. The Absence of (p)ppGpp Renders Initiation of Escherichia coli Chromosomal DNA Synthesis Independent of Growth Rates. mBio. 2020;11(2).
Related Scientific Focus Areas
Molecular Biology and Biochemistry
Genetics and Genomics
Microbiology and Infectious Diseases
This page was last updated on October 31st, 2019