Identification of the ancestral characteristics in the genome of Rhizobium leguminosarum bv. Trifolii

Nodule bacteria of the species Rhizobium leguminosarum are symbiotic N2-fixers that divide into two biotypes: viciae and t rifolii (D.C Jordan. et al., 1984). Symbiotic genes, the evolution of which depends on host plants, are responsible for the function of symbiotic nitrogen fixation (J.P.W. Young et al., 1989). Recently it was shown that according to the type of organization of the symbiotic regions of the genomes, rhizobia isolated from the Vavilovia formosa (Stev.) Fed . are close to the protosymbiont of the tribe Fabeae R. leguminosarum bv. viceae (E.R. Chirak et al., 2019). However, in the evolution of R. leguminosarum , there was another earlier divergence between the biotypes viceae and trifolii , the starting point of which was the protosymbiont of the entire species R. leguminosarum , which existed before its separation into biovars. In this work we present the results of genomes sequencing of a group of Rhizobium leguminosarum bv. trifolii and comparisons of the structure of their symbiotic regions with the corresponding regions of the genomes of Rhizobium leguminosarum bv. viciae , related to the ancestral and “advanced” types. In the program CLC Genomics Workbench 7.5.1, we compared the obtained genome-wide sequences of the strains R. leguminosarum bv. trifolii (Oxford Nanopore sequencing technique) with reference strains of R. leguminosarum bv. viceae , related to ancestral and “advanced” types. It was shown that in the genomes of strains of clover symbionts, four of five ancestral characters are found: an increased size of intergenic regions in the symbiotic region, the presence of the nodX gene in the nod-operon, the absence of the nodT gene in the sym -region, and only one copy of the fixNOPQ operon located on the pSym. Based on the results obtained, we suggest that the protosymbiont R. leguminosarum could be close to clover rhizobia.