The Sinorhizobium meliloti trans-encoded small RNA MmgR modulates PHB reserves accumulation under nutrient imbalance

LAGARES ANTONIO (JR.); CEIZEL BORELLA, GERMÁN; BECKER, ANKE; VALVERDE, CLAUDIO

Córdoba - Argentina

Congreso; XI Congreso Argentino de Microbiología General de SAMIGE; 2015

Asociación Civil de Microbiología General (SAMIGE)

Bacterial gene expression is known to be regulated at various levels. Among them, riboregulation stands for post-transcriptional tuning of expression that is orchestrated by RNA molecules. The nitrogen-fixing legume symbiont Sinorhizobium meliloti expresses ca. 450 small trans-acting non-coding RNAs with potential regulatory functions. sRNA-mediated processes may play a distinctive role in the fine-tuning of regulation of gene expression in S. meliloti, both in free-living conditions and during the symbiotic association with the plant host Medicago sativa.The S. meliloti small non-coding RNA Sm8 has been of our particular interest because it has a remote evolutionary origin and shows a high conservation among the alpha-proteobacteria. Sm8 is a trans-encoded sRNA of 77 nucleotides in length that does not codify for a protein. Sm8 is kept at basal expression levels during a nutrient-balanced saprophytic growth and begins to accumulate at the stationary phase of growth after a non-C based nutrient limitation.An S. meliloti Δsm8 mutant accumulates higher cellular biomass when growth is arrested as a consequence of imbalanced nutrient availability in the presence of an excess of a carbon source. The difference in biomass production observed between wild type and Δsm8 is due to a higher accumulation of the storage compound polyhydroxybutyrate (PHB) in the absence of Sm8. In this sense, Δsm8 is not able to limit the extent of stored PHB at increasing amounts of available C source when growth is arrested, as normally does the wild type. As expected, Sm8 expression from a plasmid gene copy in the mutant genomic background restored the wild type behavior. As further supporting evidence, ultrastructural analysis of N-starving wild type and Δsm8 stationary bacteria revealed differences in the amount and size of PHB storage granules between both strains. Transcriptomic and proteomic exploration of the Sm8 regulon revealed that Sm8 regulates -direct or indirectly- several genes known to be involved in PHB metabolism. Based on this evidence, we renamed the Sm8 transcript as MmgR, i.e., a regulatory RNA whose mutation results in a strain that makes more PHB granules. Further experiments should be conducted to explore the symbiotic relevance of MmgR as a regulator of reserves accumulation.