Plasmid pSmeLPU88b is able to entry in Ensifer meliloti LPU26 when pSmeLPU26b is absent

SALTO I.; TORRES TEJERIZO G. A.; FONDACARO L. F.; GIUSTI M. A.; LOZANO M. J.; MARTINI M. C.; SALAS M. E.; DEL PAPA M. F.; LAGARES A.; PISTORIO M.

Tucumán

Congreso; ?VII Congreso Argentino de Microbiología General, SAMIGE Del Bicentenario?; 2011

Horizontal gene transfer is mediated by several mechanisms; bacterial conjugation is the most widespread of them. Bacterial conjugation is a specialized process involving unidirectional transfer of DNA from a donor to a recipient cell by a mechanism requiring specific contact between bacteria. The genetic information for conjugation is generally encoded in a doublestranded, circular piece of DNA called plasmid. The plasmids exist in the bacterial cell entirely separated from the bacterial chromosome. Conjugation systems are very efficient in mediating transfer between a wide range of bacterial genera and, in some cases, conjugation goes beyond prokaryotes e.g. plant cells. The biology and biochemistry of plasmid transfer in soil bacteria is currently under active investigation because of its central role in prokaryote adaptation and evolution. In a previous work, we examined the conjugal properties of the cryptic plasmids present in a collection of the N2-fixing legume-symbiont Ensifer meliloti in order to investigate how frequently conjugative/mobilizable plasmids are found within the diversity of the population, and the degree of cross-complementation among the helper functions. The results show that isolates of nearly 14% of the tested strains hosted transmissible plasmids and that isolates of 29% of the tested strains were able to retransfer the previously characterized mobilizable-cryptic plasmid pSmeLPU88b to a third recipient strain. The proportion of rhizobia carrying transmissible replicons was inferred from experiments in the laboratory. It is noteworthy that isolates belonging to 14% of the tested strains proved to be refractory to the entrance of the model plasmid pSmeLPU88b, suggesting either the presence of surface exclusion phenomena or the occurrence of restriction incompatibility with the incoming replicon. In this work we approached the identification of rhizobial surface/entry exclusion functions through the Tn5-B13 mutagenesis of LPU26 strain, and the subsequent selection of those mutants that had lost the ability of exclusion of pSmeLPU88b plasmid. In one Tn5-B13 clone the entrance of pSmeLPU88b produced the lost on a resident plasmid (pSmeLPU26b). Further analysis showed that pSmeLPU26b was responsible for surface/entry exclusion.