Effects of DksA and ClpP protease on sigma S production and virulence in Salmonella typhimurium

Salmonella typhimurium responds to a variety of environmental stresses by accumulating the alternative sigma factor σ(S). The repertoire of σ(S)-dependent genes that are subsequently expressed confers tolerance to a variety of potentially lethal conditions including low pH and stationary phase. The mechanism(s) responsible for triggering σ(S) accumulation are of considerable interest, because they help to ensure survival of the organism during encounters with suboptimal environments. Two genes associated with regulating σ(S) levels in S. typhimurium have been identified. The first is clpP, encoding the protease known to be responsible for degrading σ(S) in Escherichia coli. The second is dksA, encoding a protein of unknown function not previously associated with regulating σ(S) levels. As predicted, clpP mutants accumulated large amounts of σ(S) even in log phase. However, dksA mutants failed to accumulate σ(S) in stationary phase and exhibited lower accumulation during acid shock in log phase. DksA appears to be required for the optimal translation of rpoS based upon dksA mutant effects on rpoS transcriptional and translational lacZ fusions. The region of rpoS mRNA between codons 8 and 73 is required to see the effects of dksA mutations. This distinguishes the role of DksA from that of HF-I (hfq) in rpoS translation, as the HF-I target area occurs well upstream of the rpoS start codon. DksA appears to be involved in the expression of several genes in addition to rpoS based on two-dimensional SDS-PAGE analysis of whole-cell proteins. As a result of their effects on gene expression, mutations in clpP and dksA decreased the virulence of S. typhimurium in mice, consistent with a role for σ(S) in pathogenesis.