REGULATION IN THE RPOS REGULON OF ESCHERICHIA-COLI

In Escherichia coli, the transcription factor sigma(s), encoded by rpo S, controls the expression of a large number of genes involved in cell ular responses to a diverse number of stresses, including starvation, osmotic stress, acid shock, cold shock, heat shock, oxidative DNA dama ge, and transition to stationary phase. A list of over 50 genes under the control of rpoS has been compiled. The transcription factor sigma( s) acts predominantly as a positive effector, but it does have a negat ive effect on some genes. The synthesis and accumulation of sigma(s) a re controlled by mechanisms affecting transcription, translation, prot eolysis, and the formation of the holoenzyme complex. Transcriptional control of rpoS involves guanosine 3',5'-bispyrophosphate (ppGpp) and polyphosphate as positive regulators and the cAMP receptor protein - c AMP complex (CRP-cAMP) as a negative regulator. Translation of rpoS mR NA is controlled by a cascade of interacting factors, including Hfq, H -NS, dsrA RNA, LeuO, and oxyS RNA that seem to modulate the stability of a region of secondary structure in the ribosome-binding region of t he gene's mRNA. The transcription factor sigma(s) is sensitive to prot eolysis by ClpPX in a reaction that is promoted by RssB and inhibited by the chaperone DnaK. Despite the demonstrated involvement of so many factors, arguments have been presented suggesting that sensitivity to proteolysis may be the single most important modulator of sigma(s) le vels. The activity of sigma(s) may also be modulated by trehalose and glutamate, which activate holoenzyme formation and promote holoenzyme binding to certain promoters.