RNA-seq in the broad host range strain Sinorhizobium fredii NGR234 identifies a large set of genes linked to quorum sensing-dependent regulation in the background of a traI and ngrI deletion mutant.

The alphaproteobacterium Sinorhizobium fredii NGR234 has an exceptionally wide host range as it forms nitrogen-fixing nodules with more legumes than any other known microsymbiont. Within its 6.9 Mbp genome it encodes two N-acyl-homoserine-lactone synthase genes (i.e. traI and ngrI) involved in the biosynthesis of two distinct autoinducer I-type molecules. Here we report on the construction of a NGR234-ΔtraI and a NGR234-ΔngrI mutant and their genome wide transcriptome analysis. A high-resolution RNA-seq analysis of early stationary phase cultures in the NGR234-ΔtraI background suggested that up to 316 genes were differentially expressed in the NGR234-ΔtraI mutant vs. the parent strain. Similarly, in the background of NGR234-ΔngrI 466 differentially regulated genes were identified. Accordingly, a common set of 186 genes was regulated by the TraI/R and NgrI/R regulon. Co-regulated genes included 42 flagella biosynthesis genes and 22 genes linked to exopolysaccharide (EPS) biosynthesis. Among the genes and ORFs that were differentially regulated in NGR234-ΔtraI were those linked to replication of the pNGR234a symbiotic plasmid and cytochrome c oxidases. Biotin and pyrroloquinoline quinone biosynthesis genes were differentially expressed in the NGR234-ΔngrI mutant as well as the entire cluster of 21 genes linked to assembly of the NGR234 type III secretion system (T3SS-II). Further we also discovered that genes responsible for rhizopine catabolism in NGR234 were strongly repressed in the presence of high levels of N-acyl-homoserine-lactones. Together with nodulation assays, the RNA-seq-based findings suggested that QS-dependent gene regulation appears to be of higher relevance during non-symbiotic growth rather than for life within root nodules.