Conjugal properties of the Sinorhizobium meliloti plasmid mobilome

PISTORIO, M.; GIUSTI, M.A.; DEL PAPA, M.F.; DRAGHI, W.O.; LOZANO, M.J.; TORRES TEJERIZO, G.A.; LAGARES, A.

FEMS MICROBIOLOGY ECOLOGY

Blackwell Publishing Ltd

Año: 2008 vol. 65 p. 372 - 382

0168-6496

The biology and biochemistry of plasmid transfer in soil bacteria is currently underactive investigation because of its central role in prokaryote adaptation andevolution. In this work, we examined the conjugal properties of the crypticplasmids present in a collection of the N2-fixing legume-symbiont Sinorhizobiummeliloti. The study was performed on 65 S. meliloti isolates recovered from 25humic soils of Argentina, which were grouped into 22 plasmid-profile types [i.e.plasmid operational taxonomic units (OTUs)]. The cumulative Shannon indexcalculated for the observed plasmid profiles showed a clear saturation plateau, thusindicating an adequate representation of the S. meliloti plasmid-profile types in theisolates studied. The results show that isolates of nearly 14% of the plasmid OTUshosted transmissible plasmids and that isolates of 29% of the plasmid OTUs wereable to retransfer the previously characterized mobilizable-cryptic plasmidpSmeLPU88b to a third recipient strain. It is noteworthy that isolates belongingto 14% of the plasmid OTUs proved to be refractory to the entrance of the modelplasmid pSmeLPU88b, suggesting either the presence of surface exclusion phenomenaor the occurrence of restriction incompatibility with the incomingreplicon. Incompatibility for replication between resident plasmids and plasmidpSmeLPU88b was observed in c. 20% of the OTUs. The results reported here reveala widespread compatibility among the conjugal functions of the cryptic plasmidsin S. meliloti, and this fact, together with the observed high proportion of existingdonor genotypes, points to the extrachromosomal compartment of the species asbeing an extremely active plasmid mobilome.