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 ae robic and microaerobic conditions, of Salmonella typhimurium and its isogen ic mutants nuoG::Km, cydA::TnphoA, Delta arcA and Delta rpoS was studied us ing luxAB transcriptional fusions with the rpoS and arcA genes. In the wild type strain, rpoS transcription was greater under aerobic than under microa erobic conditions, whereas transcription of arcA was suppressed by aerobios is. Under aerobic conditions, no interaction between NuoG, CydA, ArcA and R poS was detected. Under microaerobic conditions, rpoS was suppressed in the nuoG mutant as compared with the wild-type strain, but it was overexpresse d in the cydA and arcA mutants. A deletion in the rpoS gene, on the other h and, resulted in non-restricted, increased arcA expression in stationary-ph ase cultures under microaerobic conditions. Based on the rpoS transcription in the nuoG mutant the authors propose that the decrease in the NADH:NAD r atio that occurs when carbon sources become limiting serves as a signal for increased rpoS transcription, while active respiration catalysed by CydA a nd controlled by ArcA downregulates rpoS transcription. When, finally, the RpoS-controlled stationary phase of growth is reached, arcA is suppressed i n an RpoS-dependent fashion. Transition into stationary phase under microae robic conditions is thus controlled by coordinated action of the RpoS and A rcA regulators, depending on subtle changes in the environment.