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Methylation of specific DNA sequences is ubiquitous in bacteria and has known roles in immunity and regulation of cellular processes, such as the cell cycle. Here, we explored DNA methylation in bacteria of the genus Ensifer, including its potential role in regulating terminal differentiation during nitrogen-fixing symbiosis with legumes. Using single-molecule real-time sequencing, six genome-wide methylated motifs were identified across four Ensifer strains, five of which were strain-specific. Only the GANTC motif, recognized by the cell cycle-regulated CcrM methyltransferase, was methylated in all strains. In actively dividing cell cultures, methylation of GANTC motifs increased progressively from the ori to ter regions in each replicon, in agreement with a cell cycle-dependent regulation of CcrM. In contrast, there was near full genome-wide GANTC methylation in the early stage of symbiotic differentiation. This was followed by a moderate decrease in the overall extent of methylation and a progressive decrease in chromosomal GANTC methylation from the ori to ter regions in later stages of differentiation. Based on these observations, we suggest that CcrM activity is dysregulated and constitutive during terminal differentiation, which we hypothesize is a driving factor for endoreduplication of terminally differentiated bacteroids. IMPORTANCE Nitrogen fixation by rhizobia in symbiosis with legumes is economically and ecologically important. The symbiosis can involve a complex bacterial transformation-terminal differentiation-that includes major shifts in the transcriptome and cell cycle. Epigenetic regulation is an important regulatory mechanism in diverse bacteria; however, the roles of DNA methylation in rhizobia and symbiotic nitrogen fixation have been poorly investigated. We show that aside from cell cycle regulation, DNA methyltransferases are unlikely to have conserved roles in the biology of bacteria of the genus Ensifer. However, we present evidence consistent with an interpretation that the cell cycle methyltransferase CcrM is dysregulated during symbiosis, which we hypothesize may be a key factor driving the cell cycle switch in terminal differentiation required for effective symbioses.

The diversity of phage-related sequences (PRSs) and their site-specific integration into the genomes of nonpathogenic, agriculturally valuable, nitrogen-fixing root nodule bacteria, such as Sinorhizobium meliloti, were evaluated in this study. A total of 314 PRSs, ranging in size from 3.24 kb to 88.98 kb, were identified in the genomes of 27 S. meliloti strains. The amount of genetic information foreign to S. meliloti accumulated in all identified PRSs was 6.30 Mb. However, more than 53% of this information was contained in prophages (Phs) and genomic islands (GIs) integrated into genes encoding tRNAs (tRNA genes) located on the chromosomes of the rhizobial strains studied. It was found that phiLM21-like Phs were predominantly abundant in the genomes of S. meliloti strains of distant geographical origin, whereas RR1-A- and 16-3-like Phs were much less common. In addition, GIs predominantly contained fragments of phages infecting bacteria of distant taxa, while rhizobiophage-like sequences were unique. A site-specific integration analysis revealed that not all tRNA genes in S. meliloti are integration sites, but among those in which integration occurred, there were "hot spots" of integration into which either Phs or GIs were predominantly inserted. For the first time, it is shown that at these integration "hot spots", not only is the homology of attP and attB strictly preserved, but integrases in PRSs similar to those of phages infecting the Proteobacteria genera Azospirillum or Pseudomonas are also present. The data presented greatly expand the understanding of the fate of phage-related sequences in host bacterial genomes and also raise new questions about the role of phages in bacterial–phage coevolution. [ABSTRACT FROM AUTHOR]

Microbial communities associated with plants growing in harsh conditions, including salinity and water deficiency, have developed adaptive features which permit them to grow and survive under extreme environmental conditions. In the present study, an ex-situ plant trapping method has been applied to collect the culturable microbial diversity associated with the soil from harsh and remote areas. Oryza sativa cv. Baldo and Triticum durum Primadur plants were used as recruiters, while the soil surrounding the roots of Oryza glaberrima plants from remote regions of Mali (West Africa) was used as substrate for their growth. The endophytic communities recruited by the two plant species belonged to Proteobacteria and Firmicutes, and the dominant genera were Bacillus, Kosakonia , and Enterobacter. These endophytes were characterized by analyzing some of the most common plant growth promoting traits. Halotolerant, inorganic phosphate-solubilizing and N-fixing strains were found, and some of them simultaneously showing these three traits. We verified that 'Baldo' recruited mostly halotolerant and P-solubilizers endophytes, while the endophytes selected by 'Primadur' were mainly N-fixers. The applied ex-situ plant trapping method allowed to isolate endophytes with potential beneficial traits that could be applied for the improvement of rice and wheat growth under adverse environmental conditions. [ABSTRACT FROM AUTHOR]

Ensifer (syn. Sinorhizobium) meliloti is an important symbiotic bacterial species that fixes nitrogen. Strains BO21CC and AK58 were previously investigated for their substrate utilization and their plant-growth promoting abilities showing interesting features. Here, we describe the complete genome sequence and annotation of these strains. BO21CC and AK58 genomes are 6,985,065 and 6,974,333 bp long with 6,746 and 6,992 genes predicted, respectively.