Azotobacter vinelandii is a soil bacterium that produces the polysaccharide alginate. The two-component system GacS/GacA is required for alginate synthesis since a mutation in gacS or gacA significantly reduced the level of transcripts of algD, the gene encoding GDP-mannose dehydrogenase, a key enzyme of the alginate biosynthetic pathway. In many γ-proteobacteria, GacA homologs control the expression of small regulatory RNAs of the RsmZ/Y/X (CsrB/CsrC) family that interact with RsmA (CsrA) proteins. These proteins bind to their target mRNAs acting as translational repressors. The interaction of Rsm/Csr small RNAs with RsmA/CsrA counteract its repressor activity. In this study, one rsmA gene, seven rsmZ and two rsmY homologs were identified in the A. vinelandii genome. Two of the rsmZ homologs, named rsmZ1 and rsmZ2, together with rsmA, were characterized. Northern blot analysis was carried out to show that in A. vinelandii, GacA activates rsmZ1 and rsmZ2 transcription. We also showed that either overexpression of rsmA or inactivation of rsmZ1 or rsmZ2 diminished the production of alginate. In addition, interaction of RsmA with RsmZ1, RsmZ2 and the algD mRNA was demonstrated in vitro. These results show that GacS/A regulates alginate biosynthesis by post-transcriptional control of algD expression through the Rsm system.
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