The Sinorhizobium medicae WSM419 IpiA gene is transcriptionally activated by FsrR and required to enhance survival in lethal acid conditions

Sinorhizobium medicae WR101 was identified as a mutant of WSM419 that contained a minitransposon-induced transcriptional gusA fusion activated at least 20-fold at pH 5-7. The expression of this fusion in moderately acid conditions was dependent on the calcium concentration; increasing the calcium concentration to enhance cell growth and survival in acid conditions decreased the expression of the fusion. A gene region containing the gusA fusion was sequenced, revealing five S medicae genes: tcsA, tcrA, fsrR, lpiA and acvB. The gusA reporter in WRI101 was fused to lpiA, which encodes a putative transmembrane protein also found in other Alphaproteobacteria such as Sinorhizobium metiloti, Rhizobium tropici and Agrobacterium tumetaciens. As LpiA has partial sequence similarity to the lysyl-phosphatidylglycerol (LPG) synthetase FmtC/MprF from Staphylococcus aureus, membrane lipid compositions of S. medicae strains were analysed. Cells cultured under neutral or acidic growth conditions did not induce any detectable LPG and therefore this lipid cannot be a major constituent of S. medicae membranes. Expression studies in S. medicae localized the acid-activated IpiA promoter within a 372 bp region upstream of the start codon. The acid-activated transcription of 1piA required the fused sensor-regulator product of the fsrR gene, because expression of lpiA was severely reduced in an S. medicae fsrR mutant. S. meliloti strain 1021 does not contain fsrR and acid-activated expression of the lpiA-gusA fusion did not occur in this species. Although acid-activated 1piA transcription was not required for cell growth, its expression was crucial in enhancing the viability of cells subsequently exposed to lethal acid (pH 4.5) conditions. © 2006 SGM.