Transcription of arcA and rpoS during growth of Salmonella typhimurium under aerobic and microaerobic conditions.

Physiology of the exponential and stationary phase of growth, under both aerobic and microaerobic conditions, of Salmonella typhimurium and its isogenic mutants nuoG::Km, cydA::TnphoA, DeltaarcA and DeltarpoS was studied using luxAB transcriptional fusions with the rpoS and arcA genes. In the wild-type strain, rpoS transcription was greater under aerobic than under microaerobic conditions, whereas transcription of arcA was suppressed by aerobiosis. Under aerobic conditions, no interaction between NuoG, CydA, ArcA and RpoS was detected. Under microaerobic conditions, rpoS was suppressed in the nuoG mutant as compared with the wild-type strain, but it was overexpressed in the cydA and arcA mutants. A deletion in the rpoS gene, on the other hand, resulted in non-restricted, increased arcA expression in stationary-phase cultures under microaerobic conditions. Based on the rpoS transcription in the nuoG mutant the authors propose that the decrease in the NADH:NAD ratio that occurs when carbon sources become limiting serves as a signal for increased rpoS transcription, while active respiration catalysed by CydA and controlled by ArcA downregulates rpoS transcription. When, finally, the RpoS-controlled stationary phase of growth is reached, arcA is suppressed in an RpoS-dependent fashion. Transition into stationary phase under microaerobic conditions is thus controlled by coordinated action of the RpoS and ArcA regulators, depending on subtle changes in the environment.