The General Stress Sigma Factor ς(S) of Escherichia coli Is Induced during Diauxic Shift from Glucose to Lactose

The general stress sigma factor ς(S) (RpoS) of Escherichia coli is strongly induced in response to glucose starvation. This increase in the cellular ς(S) level is due to stabilization of ς(S), which under non-stress conditions is subject to rapid proteolysis. In the present study, it is demonstrated that ς(S) is also induced during the diauxic shift from glucose to lactose, i.e., under conditions of glucose exhaustion in the presence of another, less-preferred carbon source that eventually gets utilized. This ς(S) induction, which is due to stabilization, is transient and precedes the induction of β-galactosidase. In parallel, ς(S)-dependent genes are transiently activated, as was shown here for osmY. Although ς(S) can mediate transcription of lacZ in vitro, ς(S) does not contribute to the induction of β-galactosidase during the diauxic lag phase. Rather, the induction of ς(S) and the general stress response during the diauxic shift plays the role of a rapidly activated emergency system, which is shut off again as soon as the cells are able to cope with the stress situation by utilizing a more specific and more economical system.