ς(70) Is the Principal Sigma Factor Responsible for Transcription of acs, Which Encodes Acetyl Coenzyme A Synthetase in Escherichia coli

Cells of Escherichia coli undergo a metabolic switch associated with the production and utilization of acetate. During exponential growth on tryptone broth, these cells excrete acetate via the phosphotransacetylase-acetate kinase (Pta-AckA) pathway. As they begin the transition to stationary phase, they instead resorb acetate, activate it to acetyl coenzyme A (acetyl-CoA) by means of the enzyme acetyl-CoA synthetase (Acs) and utilize it to generate energy and biosynthetic components via the tricarboxylic acid cycle and the glyoxylate shunt, respectively. This metabolic switch depends upon the induction of Acs. As part of our effort to dissect the mechanism(s) underlying induction and to identify the signal(s) that triggers that induction, we sought the sigma factor most responsible for acs expression. Using isogenic strains that carry a temperature sensitivity allele of the gene that encodes ς(70) and either a wild-type or null allele of the gene that encodes ς(S), we determined by immunoblotting, reverse transcriptase PCR, and acs::lacZ transcriptional fusion analyses that ς(70) is the sigma factor primarily responsible for the acs transcription that cells induce during mid-exponential phase. In contrast, ς(S) partially inhibits that transcription as cells enter stationary phase.