User:Rcrzarg/Small non-coding RNAs in the legume Sinorhizobium meliloti

Introduction
Post-genomic research has rendered bacterial small non-coding RNAs (sRNAs) as major players in post-transcriptional regulation of gene expression in response to environmental stimuli. The α-subdivision of the Proteobacteria includes Gram-negative microorganisms with diverse life styles; frequently involving long-term interactions with higher eukaryotes.

Sinorhizobium meliloti
Sinorhizobium meliloti is a α-proteobacteria representative able to induce the formation of new specialized organs, the so-called nodules, in the roots of its cognate legume hosts. Within the nodule cells bacteria undergo a morphological differentiation to bacteroid, their endosymbiotic nitrogen-fixing competent form. S. meliloti is an agronomically relevant microorganism that establishes a nitrogen-fixing endosymbiosis with various forage legumes, including alfalfa (Medicago sativa L). In the proximity of the root hairs, the plant flavone luteolin specifically induces the synthesis and secretion of lipo-quitooligosaccharide signal molecules (Nod factors) in S. meliloti upon the transcriptional activation of the nodulation (Nod) genes by the NodD1/NodD2 proteins. Subsequently, bacterial Nod factors trigger infection and organogenesis of new specialized organs in the plant, the so-called root nodules, where the microsymbiont differentiates into its nitrogen-fixing competent form, the bacteroid, within the plant cell. Rhizobial adaptations to soil and plant cell environments require the coordinate expression of complex gene networks in which sRNAs are expected to participate.

Discovery
Two complementary strategies, eQRNA and RNAz, were used to search for novel sRNA-encoding genes in the IGRs of S. meliloti. Verification of eQRNA/RNAz predictions by Northern hybridization and RACE mapping led to the identification of eight previously unknown loci expressing small transcripts and organized in independent transcription units. Seven of the identified sRNAs are differentially regulated in free-living and symbiotic bacteria, which predicts novel regulatory functions for bacterial sRNAs in the α-proteobacteria–eukaryotes interactions.