The α-proteobacterial trans-encoded small RNA MmgR: Evolutionary traits and its regulatory role on polyhydroxybutyrate accumulation in Sinorhizobium meliloti

LAGARES ANTONIO (JR.); CEIZEL BORELLA, GERMÁN; ROUX, INDRA; UWE LINNE; BECKER, ANKE; VALVERDE, CLAUDIO

Rosario - Argentina

Congreso; Congreso Latinoamericano y Argentino de Microbiología 2016 ? ALAM-AAM; 2016

Asociación Latinoamericana de Microbiología y Asociación Argentina de Microbiología

Riboregulation in bacteria has a major role in the fine-tuning of multiple cellular processes. In the nitrogen fixing alfalfa symbiont Sinorhizobium meliloti, hundreds of trans-encoded untranslated small RNAs (sRNAs) with potential regulatory roles are expressed under different environmental and growth conditions. Nevertheless, with a few exceptions, their biological implications remain largely unexplored.We have performed a comprehensive characterization of the biological role of the S. meliloti sRNA MmgR -formerly known as Sm8- in frame with the study of its evolutionary and phylogenetic aspects. In S. meliloti, MmgR is a 77-nt long sRNA whose expression is highly induced upon nitrogen limitation and carbon surplus during free living growth, and in mature nodule bacteroids during the interaction of the rhizobia with the host plant.An initial search for MmgR homologs in bacterial genomes -based on both sequence and RNA structure conservation- revealed that MmgR evolves from a common ancestor of the α-proteobacteria that diverged from the order of Rickettsiales, being thus so far the sRNA with the widest phylogenetic distribution among this bacterial clade. Further analyses allowed us to identify a strong genetic linkage between a set of mmgR homologs and homologs of a protein coding gene, letting us trace the vertical evolutionary path of this group of orthologs and recognize sequence and structure traits that have been completely conserved throughout their evolution.The characterization of MmgR function in S. meliloti was carried out following a reverse genetics approach. We found that MmgR is a negative regulator of the accumulation of the major carbon and reducing power storage polymer polyhydroxybutyrate (PHB) in S. meliloti cells growing under conditions of C:N overbalance. Our results suggest that MmgR expression in S. meliloti sets a limit for PHB accumulation under N starvation and C surplus. An exploration of the molecular bases of MmgR-mediated regulation of PHB accumulation that was carried out by differential quantitative proteomic profiling and qRT-PCR analyses, provided strong evidences supporting a post-transcriptional regulation by MmgR of both PHB-granule associated phasin proteins. So far, no clear symbiotic phenotype was found upon mutation or overexpression of MmgR.The genetic capacity to synthesize and store PHB seems to be broadly distributed in α-proteobacteria, provided representatives from all orders ? except for the Rickettsiales ? have been reported to metabolize PHB. Hence, the occurrence of mmgR alleles parallels the phylogenetic distribution of PHB metabolism within all orders in α-proteobacteria, and suggests a deep co-evolutionary linkage between mmgR and PHB genetic elements. Further studies on the functional adaptation of each mmgR ortholog to its corresponding species biology will help to comprehensively understand the biological role of the most ancient and distributed sRNA gene in α-proteobacteria.