hns, rpoS and lrp mutations affect stationary-phase survival at high osmolarity.

The survival of enteric bacteria during stationary phase requires the expressions of different genes than those required for growth during log phase. Genes coding for functions protecting cells from environmental stress are expressed during the onset of stationary phase. Many promoters of these genes require sigma s, the product of the rpoS gene, for transcription. During stationary phase, we found that strains lacking the neutral, histone-like, DNA-binding protein H-NS (hns- rpoS- double mutants) are more viable at high osmolarity at 37 degrees C than hns+ rpoS- cells. We did not observe differential viability at high osmolarity at 30 degrees C or at normal osmolarity at 37 degrees C. We showed that this effect is due to the absence of H-NS. The leucine-responsive regulatory protein acts without H-NS to protect a strain containing an rpoS mutation from death in stationary phase at high osmolarity. We present evidence that hns- rpoS double mutants can synthesize the log phase osmotic shock protective system.