Genes involved in Sec-independent membrane targeting of hydrogenase in Azotobacter chroococcum

Sec-independent translocation systems have been characterised in Escherichia coli and other bacteria and differ from the Sec-dependent system by transporting fully folded proteins using the transmembrane proton electrochemical gradient. Proteins transported by this system bear a twin-arginine motif (tat) in the N-terminal signal peptide and include several cofactor-containing proteins. Azotobacter chroococcum strain (MCD124) has a soluble hydrogenase, which exhibited low O2-dependent H2 uptake, and a shift in the pH of the culture to a more alkaline range during growth. We show that the DNA region capable of complementing this strain contains the tatABC genes and that mutations in the tatA gene reproduced the soluble hydrogenase and the culture pH shift phenotypes. We also show that insertional mutation in the tatC gene at a position corresponding to its C-terminal region had no effect on hydrogenase activity, but induced the pH shift of the culture. Sequence and mutagenesis analyses of this genomic region suggest that these genes form an operon that does not contain a tatD-like gene. A mutation in hupZ of the main hup gene region, coding for a possible b-type cytochrome also yielded a soluble hydrogenase, but not the pH-shift phenotype.