Depletion of BamA activates the RSI circuit in S. meliloti

The symbiotic relationship between the alpha-proteobacterium S. meliloti and the legume M. truncatula has been extensively studied. However, stress response mechanisms vital to bacterial survival and adaptation are as yet poorly understood. In this study, we sought to characterize the response of S. meliloti to depletion of essential outer membrane protein BamA, which is predicted to cause a loss of outer membrane integrity and an accumulation of unfolded proteins in the periplasm. A total of 132 differentially expressed genes were detected, with 74 genes up-regulated and 58 genes down-regulated. The transcriptomic response to BamA depletion shares similarities to those observed during activation of the RSI circuit, which plays a role in mitigating loss of viability in response to this stress. Induction of genes in the RSI regulon is likely due to a loss of negative regulator ExoR during BamA depletion. The two-component stress response system CpxRA and regulator CpxP also were found to affect the response to BamA depletion. To identify genes whose expression changes upon depletion of BamA, we used custom affymetrix stSmel_Mtra GeneChips to compare CL150 ΔbamA with bamA on lac-inducible vector pSRKKm (Khan et al 2008; Applied and Environmental Microbiology) grown with and without 1mM IPTG for 4 hours. Three replicates were performed for each condition.