snr-1 gene is required for nitrate reduction in Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa is able to use nitrate for both assimilation and anaerobic respiration. One set of genes, designated snr (for "shared nitrate reduction"), have been recently cloned and partially characterized. In this study, we demonstrate that the snr-1 gene encodes a predicted 52.5-kDa protein that is 82% similar to a unique cytochrome c of Desulfomonile tiedjei DCB-1. Importantly, the Snr-1 protein sequence of P. aeruginosa differed from that of the cytochrome c of D. tiedjei primarily in the first 25 amino acids, which are required for membrane attachment in D. tiedjei. In P. aeruginosa, the Snr-1 protein hydropathy profile indicates that it is a soluble protein. An isogenic snr-1::Gm insertional mutant was unable to grow aerobically with nitrate as a sole nitrogen source or anaerobically with nitrate as an electron acceptor. Complementation of the snr-1::Gm mutant with the snr-1 gene restored the wild-type phenotypes. Interestingly, anaerobic growth rates were significantly higher in the snr-1 mutant harboring a multicopy plasmid containing snr-1. In contrast, aerobic growth rates of the restored mutant using nitrate as the sole nitrogen source were similar to those of the wild type. Transcriptional lacZ fusions demonstrated that snr-1 was not regulated by molybdate, oxygen, or nitrate.