ABUNDANCE OF PHAGE-RELATED SEQUENCES ON NON-SYMBIOTIC PLASMIDS OF SINORHIZOBIUM MELILOTI FROM CENTERS OF LEGUME PLANTS DIVERSITY

Abstract

Genomes of alfalfa root nodule bacteria, Sinorhizobium meliloti, symbionts of alfalfa are enriched in non-symbiotic (cryptic) plasmids, which gene pool is remained weakly studied. S. meliloti strains are significantly varied in number and size of these plasmids. The goal of the study was to assess the occurrence of phage-related sequences (PRS) on cryptic plasmids. Whole genome sequences of 12 S. meliloti strains native to Caucasian and Kazakhstan centers of alfalfa diversity (NCG and PAG, correspondingly) were studied and 20 cryptic plasmids, which sizes varied from 17.2 to 453.8 kb, were assembled. In total 55 PRS were identified on cryptic plasmids, and these sequences were represented by intact, questionable and incomplete sequences according to PHASTER. Significant differences in the occurrence of above-mentioned types of PRS on cryptic plasmids was detected between strains native to NCG and PAG (X2 = 6.73, p = 0.03). The sizes of the desired PRS varied from 5.1 to 33 kb, and their number was from 1 to 11 per replicon in tested strains. It was revealed that PRS on plasmids of strains from NCG were predominantly related to Siphoviridae family (p smaller than 0.05), while PRS homologous to phages of Siphoviridae and Podoviridae families prevailed with equal frequencies on plasmids of strains from PAG. For 40% of tested PRS the attL/attR sequences were detected and that is proving their site-specific integration type. ORFs of PRS as it was revealed are encoded integrases, fiber protein and tail shaft, and nearly all PRS are contained ORFs encoded transposases. Summarizing, S. meliloti strains native to origins of alfalfa diversity are enriched in cryptic plasmids, and the latest are attractive for soil bacteriophages, that is strongly evident the participation of small size plasmids in horizontal gene transfer process.