Expression of the thioredoxin gene (trxA) in Rhodobacter sphaeroides Y is regulated by oxygen

The structural gene (trxA) coding for thioredoxin in the photosynthetic bacterium Rhodobacter sphaeroides has been cloned and sequenced previously. In the present study, the role of oxygen in trxA expression in R. sphaeroides Y was investigated using mRNA analyses and plasmid-borne trxA'-lacZ+ translational and transcriptional fusions. Northern analysis revealed a trxA-specific transcript of approximately 420-460 nucleotides, indicating that trxA is transcribed as a single gene. By studying the beta-galactosidase activity in strains harboring various phi(trxA'-lacZ+) fusion constructs, the promoter region of the trxA gene was localized within a 64-bp region located 97 nucleotides upstream of the trxA initiator codon. A single trxA transcription initiation site was mapped by primer extension, 27 bp upstream of the trxA gene. Based on these results and the DNA sequence analysis, we propose that a sigma70 consensus sequence serves as a trxA promoter. Results from oxygen shift experiments, as deduced from both mRNA analysis and fusions of the trxA promoter region to lacZ indicate that transcription of the R. sphaeroides trxA gene is regulated by high oxygen tension. DNA sequences involved in this oxygen regulation were also localized in the 64-bp region containing the trxA promoter. Based on our findings the hypothetical biological function of thioredoxin from R. sphaeroides is discussed.