Abstract
Evidences for an involvement of the bacterial type IV secretion system (T4SS) in the symbiotic relationship between rhizobia and legumes have been pointed out by several recent studies. However, information regarding this secretion system in Mesorhizobium is still very scarce. The aim of the present study was to investigate the phylogeny and expression of the traG gene, which encodes a substrate receptor of the T4SS. In addition, the occurrence and genomic context of this and other T4SS genes, namely, genes from tra/trb and virB/virD4 complexes, were also analyzed in order to unveil the structural and functional organization of T4SS in mesorhizobia. The location of the T4SS genes in the symbiotic region of the analyzed rhizobial genomes, along with the traG phylogeny, suggests that T4SS genes could be horizontally transferred together with the symbiosis genes. Regarding the T4SS structural organization in Mesorhizobium, the virB/virD4 genes were absent in all chickpea (Cicer arietinum L.) microsymbionts and in the Lotus symbiont Mesorhizobium japonicum MAFF303099T. Interestingly, the presence of genes belonging to another secretion system (T3SS) was restricted to these strains lacking the virB/virD4 genes. The traG gene expression was detected in M. mediterraneum Ca36T and M. ciceri LMS-1 strains when exposed to chickpea root exudates and also in the early nodules formed by M. mediterraneum Ca36T, but not in older nodules. This study contributes to a better understanding of the importance of T4SS in mutualistic symbiotic bacteria.
Highlights
Rhizobia are able to fix atmospheric nitrogen when in symbiosis with legumes, providing ammonia to these plants
The nodD, nifA, and traG nucleotide sequences of these genes were analyzed for 33 mesorhizobia strains able to nodulate different plant hosts (Anthyllis vulneraria, Biserrula pelecinus, Bituminaria bituminosa, Cicer arietinum, Glycine max, Lotononis carinata, Lotus spp., and Medicago sativa) (Figures 1, 2, and 3, respectively)
Three main clusters of mesorhizobia strains could be distinguished: mesorhizobia able to nodulate Cicer arietinum; strains that were isolated from Biserrula pelecinus grouping closer with a strain nodulating Anthyllis vulneraria; and mesorhizobia able to nodulate several Lotus species
Summary
Rhizobia are able to fix atmospheric nitrogen when in symbiosis with legumes, providing ammonia to these plants. Nitrogen fixation genes include genes involved in the synthesis, processing, and assembly of nitrogenase complex (e.g., nifHDK, fixGH), responsible for N2-fixation [2,3,4]. The expression of these sets of genes is regulated by two key transcriptional regulators, namely, NodD for nodulation genes and NifA for nitrogen fixation genes [3, 5]. Some studies have shown that genes commonly found among prokaryotes and involved in a diversity of cellular mechanisms have a role in legume-Rhizobium symbiosis, as, for example, stress response genes [6,7,8], quorum sensing, or secretion system genes [9,10,11,12]
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