Identifying the regulon of the Sinorhizobium meliloti JspA protease

Sinorhizobium meliloti establishes symbiotic relationship with compatible leguminous plants by inducing root nodule formation, colonizing such nodules, and fixing molecular nitrogen for the host in exchange for carbon compounds. This mutualistic process requires complex communication and tight regulation to allow yet constrain infection to specific tissues. Production of succinoglycan, or exopolysaccharide-I (EPS-I), enables S. meliloti to invade the root cortex via infection threads. A previous genetic screen identified jspA (SMc03872), encoding an extracytoplasmic protease, as a regulator of EPS-I production. To elucidate its molecular role, we performed transcriptome analyses of strains overexpressing wild-type or mutant alleles of jspA. We observed changes in gene expression suggesting that JspA contributes to symbiosis efficiency by modulating the critical ExoR-ExoS-ChvI signaling pathway. To identify genes whose expression depends on a functional JspA protease, we used custom Affymetrix GeneChips to compare the transcriptomes of three strains: wild-type S. meliloti Rm1021 ectopically expressing jspA (JOE4140) from a taurine-inducible promoter (PtauA: Mostafavi et al., 2014; BMC Microbiol 14: 295) on a plasmid (pJC535), a similar strain expressing a mutant jspA allele (with E148A mutation in the protease active site) (JOE4400, carrying pJC555), and a reference strain carrying the empty vector, pCM130 (JOE3200). Three biological replicates were performed for each of the three strains.