Differential expression of NiFe uptake-type hydrogenase genes in Salmonella enterica serovar Typhimurium

Salmonella enterica serovar Typhimurium possesses three similar NiFe hydrogenases important to its virulence. Here we show that the three hydrogenase operons hyb, hya and hyd are expressed under different environmental conditions and are subject to control by different regulatory proteins. Hydrogenase promoter-lacZ fusion plasmids were transferred into the wild-type strain or into arcA, fnr, iscR, narL and narP deletion mutants, or into a fnr/arcA double mutant. The hyb promoter had highest beta-galactosidase activity under growth conditions promoting anaerobic respiration (glycerol plus fumarate) and may be subject to glucose repression, since cells grown with glucose had about half the transcriptional activity of cells grown with mannose. Based on the phenotype of regulatory mutant strains, IscR represses hyb aerobically, and ArcA plays a role in both hyb and hyd regulation. The hyd promoter had about five times more activity in cells grown under aerobic conditions compared to anaerobic levels, and its activity tripled in an arcA mutant grown anaerobically. The hya promoter had the highest activity when cells were grown anaerobically with glucose, and the growth yield of the hya mutant was about 25 % lower than for wild-type cells grown fermentatively, suggesting that Hya may be utilized during fermentation. The hya promoter is repressed by nitrate and this repression was abolished when the NarL-binding site was mutated, or in a narL mutant background. FNR is involved in hyb and hya regulation, since beta-galactosidase activity decreased significantly in a fnr mutant. These findings suggest that the three hydrogenases are used under different conditions, likely enhancing the pathogen's capacity to survive in a variety of environments.