Transcriptional mechanisms for differential expression of outer membrane cytochrome genes omcA and mtrC in Shewanella oneidensis MR-1

Background: Shewanella oneidensis MR-1 is capable of reducing extracellular electron acceptors, such as metals and electrodes, through the Mtr respiratory pathway, which consists of the outer membrane cytochromes OmcA and MtrC and associated proteins MtrA and MtrB. These proteins are encoded in the mtr gene cluster (omcA-mtrCAB) in the MR-1 chromosome. Results: Here, we investigated the transcriptional mechanisms for the mtr genes and demonstrated that omcA and mtrC are transcribed from two upstream promoters, P-omcA and P-mtrC, respectively. In vivo transcription and in vitro electrophoretic mobility shift assays revealed that a cAMP receptor protein (CRP) positively regulates the expression of the mtr genes by binding to the upstream regions of P-omcA and P-mtrC. However, the expression of omcA and mtrC was differentially regulated in response to culture conditions; specifically, the expression from P-mtrC was higher under aerobic conditions than that under anaerobic conditions with fumarate as an electron acceptor, whereas expression from P-omcA exhibited the opposite trend. Deletion of the region upstream of the CRP-binding site of P-omcA resulted in a significant increase in promoter activity under aerobic conditions, demonstrating that the deleted region is involved in the negative regulation of P-omcA. Conclusions: Taken together, the present results indicate that transcription of the mtr genes is regulated by multiple promoters and regulatory systems, including the CRP/cAMP-dependent regulatory system and yet-unidentified negative regulators.