A CsrA/RsmA translational regulator gene encoded in the replication region of a Sinorhizobium meliloti cryptic plasmid complements Pseudomonas fluorescens rsmA/E mutants

BETINA AGARAS; PATRICIO SOBRERO; CLAUDIO VALVERDE

MICROBIOLOGY-UK

SOC GENERAL MICROBIOLOGY

Lugar: London; Año: 2013 vol. 159 p. 230 - 242

1350-0872

Members of the CsrA/RsmA family are global regulatory proteins that bind to mRNAs usually at the ribosome binding site to control mRNA translation and stability. Their activity is counteracted by small non coding RNAs (sRNAs) that offer several binding sites to compete with mRNA binding. csrA/rsmA genes are widespread in prokaryotic chromosomes although certain phylogenetic groups like a-proteobacteria lack this type of global regulators. Interestingly, a csrA/rsmA-like sequence was identified in the replication region of plasmid pMBA19a from the a-proteobacterium Sinorhizobium meliloti. This rsmA-like allele (rsmASm) is 58% identical to Xanthomonas axonopodis pv. citri chromosomal rsmA and bears an unusual C-terminal extension that may fold into an extra a-helix. Homology-based modelling of RsmASm suggests that all key mRNA binding residues are conserved and correctly positioned in the RNA binding pocket. In fact, a 1.6 kb fragment from pMBA19a encompassing the rsmASm locus restored rsmA/E-dependent phenotypes of rsmA/E gacS P. fluorescens mutants. The functionality of RsmASm was confirmed by the gain of control over target aprA'- and hcnA'-'lacZ translational fusions in the same mutant background. The RsmASm activity correlated with Western blot detection of the polypeptide. Phenotype and translational fusion data from rsmA/E P. fluorescens mutants expressing RsmX/Y/Z RNAs indicated that RsmASm is able to bind these antagonistic sRNAs. In agreement with the latter observation, it was also found that the sRNA RsmY was stabilized by RsmASm. Deletion of the C-terminal extra -helix of RsmASm affected its cellular concentration, but increased its relative RNA binding activity. This is the first report of the presence and characterization of a functional csrA/rsmA homolog in a mobile genetic element.