Evaluation of Sinorhizobium meliloti Genomic Islands Inserted into the tRNA-Thr

Transfer RNAs (tRNA) widely abundant in genomes are extremely conservative molecules with well-defined role in protein synthesis. The central role of tRNA in the evolution of the genetic code and theirs novel undiscovered functions were recently predicted: these sequences could be a hotspots for the integration of foreign genomic DNA, like phage related genomic islands (GIs). In this work the 17th GIs site-specifically integrated into isoacceptors tRNAs-Thr (isotRNA-Thr) out of 84th GIs detected in genomes of Sinorhizobium meliloti strains were evaluated. It was shown that the homology of entire sequences of GIs inserted into a particular tRNA isoacceptor in genetically related strains is more than 80%, whereas homology of similar types of GIs in native strains from one population or from geographically remote areas could be distinct up to 70%. For the first time CRISPRs sequences were revealed in some GIs of S. meliloti. The phylogenetic analysis done for integrase encoding sequences of distinct GIs inserted in iso-tRNAs-Thr showed their homology with the corresponding sequences of phages from Podovirida, Myoviridae and Siphoviridae families. Proceeded data allowed concluding that the insertion of GIs into precise tRNA-Tre isoaceptor occurred due to the att-homologous recombination mechanism and that GIs of S. meliloti could be involved in improving adaptive immunity of rhizobia.