PROTEASES AND THEIR TARGETS IN ESCHERICHIA-COLI

Proteolysis in Escherichia coli serves to rid the cell of abnormal and misfolded proteins and to limit the time and amounts of availability of critical regulatory proteins. Most intracellular proteolysis is ini tiated by energy-dependent proteases, including Lon, ClpXP, and HflB; HflB is the only essential E. coli protease. The ATPase domains of the se proteases mediate substrate recognition. Recognition elements in ta rget are not well defined, but are probably not specific amino acid se quences. Naturally unstable protein substrates include the regulatory sigma factors for heat shock and stationary phase gene expression, sig ma 32 and RpoS. Other cellular proteins serve as environmental sensors that modulate the availability of the unstable proteins to the protea ses, resulting in rapid changes in sigma factor levels and therefore i n gene transcription. Many of the specific proteases found in E. coli are well-conserved in both prokaryotes and eukaryotes, and serve criti cal functions in developmental systems.