Plasmid electroporation of Sinorhizobium strains: The role of the restriction gene hsdR in type strain Rm1021

Although horizontal gene transfer mediated by plasmids is important to the generation of the genetic variability of Sinorhizobium strains, the barriers which can reduce horizontal gene transfer between bacteria have not yet been studied in Sinorhizobium. We studied the plasmid transfer by electroporation and its restriction in strains of Sinorhizobium meliloti and S. medicae. After conditions for electroporation were established, three S. meliloti strains (including the sequenced type strain Rm1021) and two S. medicae strains were electroporated with plasmid DNA extracted from strains of both species. The efficiency of transformation was found to be variable among different strains. The acquisition of plasmid DNA was found to be donor-dependent in S. meliloti strain Rm1021 that prefers self-DNA more than the DNA from other Sinorhizobium strains. All other strains tested did not show a preference for self-DNA. In strain Rm1021, the inactivation of the hsdR gene, coding for a putative type-I restriction enzyme, increased the efficiency of transformation and conjugation with non-self DNA; the transformation capability was again reduced in hsdR mutant when the cloned hsdR gene was expressed from a lac promoter. Phylogenetic analysis of the hsdR gene clearly indicated that this gene was horizontally transferred to strain Rm1021, explaining its absence in the other strains tested.