Charles Michael Cashel, M.D.,Ph.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, commonly abbreviated as (p)ppGpp. These are analogs of GTP and GDP with ribosyl 3' pyrophosphate residues; the compounds function as important second messengers in bacteria and plants. A generalized nutritional stress results in an increase in cellular levels of ppGpp. Examples of effective nutritional stress for ppGpp regulation include starvation for sources of nitrogen, carbon, amino acids, phosphate, iron, oxygen, or specific vitamins (1). 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 level. 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 (4). The details of these global mechanisms at the level of transcription and whether there are additional nucleotide analogs with similar functions continue to hold our central interest, and we use molecular microbiology and genetics as the most tractable approach (2, 3).
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.
Fernández-Coll L, Cashel M. Contributions of SpoT Hydrolase, SpoT Synthetase, and RelA Synthetase to Carbon Source Diauxic Growth Transitions in <i>Escherichia coli</i>. Front Microbiol. 2018;9:1802.
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.
Molodtsov V, Sineva E, Zhang L, Huang X, Cashel M, Ades SE, Murakami KS. Allosteric Effector ppGpp Potentiates the Inhibition of Transcript Initiation by DksA. Mol Cell. 2018;69(5):828-839.e5.
Bruhn-Olszewska B, Molodtsov V, Sobala M, Dylewski M, Murakami KS, Cashel M, Potrykus K. Structure-function comparisons of (p)ppApp vs (p)ppGpp for Escherichia coli RNA polymerase binding sites and for rrnB P1 promoter regulatory responses in vitro. Biochim Biophys Acta Gene Regul Mech. 2018;1861(8):731-742.
Related Scientific Focus Areas
Molecular Biology and Biochemistry
Genetics and Genomics
This page was last updated on October 31st, 2018