Role of Salmonella enterica Serovar Typhimurium Two-Component System PreA/PreB in Modulating PmrA-Regulated Gene Transcription

ThePmrA/PmrB two-component system encoded by thepmrCAB operon regulates the modification ofSalmonellaenterica serovar Typhimurium lipopolysaccharide leading topolymyxin B resistance. PmrA and PhoP are the only known activators ofpmrCAB . A transposon mutagenesis screen for additionalregulators of apmrC ::MudJ fusion led tothe identification of a two-component system, termed PreA/PreB(pmrCAB regulators A and B), that controls the transcriptionof thepmrCAB operon in response to unknown signals. Theinitial observations indicated that insertions in, or a deletion of,thepreB sensor, but not thepreA response regulator,caused upregulation ofpmrCAB . Interestingly, theexpression ofpmrCAB was not upregulated in apreAB mutant grown in LB broth, implicating PreA in the increased expressionofpmrCAB in thepreB strain. This was confirmed byoverexpression ofpreA + inpreAB orpreB backgrounds, which resulted insignificant upregulation or further upregulation ofpmrCAB . Nosuch effect was observed in any testedpreB + backgrounds. Additionally,an ectopic construct expressing apreA [D51A] allele alsofailed to upregulatepmrC in any of thepre backgrounds tested, which implies that there is a need forphosphorylation in the activation of the target genes. The observedupregulation ofpmrCAB occurred independently of the responseregulators PmrA and PhoP. Although apreB mutation led toincreased transcription ofpmrCAB , this did not result in ameasurable effect on polymyxin B resistance. Our genetic data support amodel of regulation whereby, in response to unknown signals, the PreBsensor activates PreA, which in turn indirectly upregulatespmrCAB transcription.