The cryptic-plasmid mobilome in the n2-fixing soil-bacteria Sinorhizobium meliloti

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

Gdansk, Polonia

Congreso; International Plamid Biology Conference (IPBC 2008); 2008

International Society for Plasmid Biology

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 this work, we examined the conjugal properties of the cryptic plasmids present in a collection of the N2-fixing legume-symbiont Sinorhizobium meliloti in order to investigate how frequently conjugative/mobilizable plasmids are found within the collected diversity within the species, and the degree of cross-complementation among the helper functions. The study was performed on 65 S. meliloti isolates recovered from 25 humic soils of Argentina, which were grouped into 22 plasmid-profile types (i.e., plasmid OTUs-Operational Taxonomic Units). The cumulative Shannon index calculated for the observed plasmid profiles showed a clear saturation plateau for n>49 (n = number of isolates used for the calculation), thus indicating an adequate representation of the S. meliloti plasmid-profile types in the isolates studied. The results show that isolates of nearly 14% of the plasmid OTUs hosted transmissible plasmids and that isolates of 29% of the plasmid OTUs were able to retransfer the previously characterized mobilizable-cryptic plasmid pSmeLPU88b (Pistorio et al., 2003) 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 plasmid OTUs 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. Incompatibility for replication between resident plasmids and plasmid pSmeLPU88b was observed in ca. 20% of the OTUs. We have previously reported that plasmid pSmeLPU88b carries two functional plasmid replication modules, one of them belonging to the RepABC-type and the other related to the type A family of replication initiation proteins (Pistorio et al., ISPB 2006). Interestingly, the type A RepC protein showed more than two-hundred additional amino acids at its C-terminal end compared to other RepC homologs from related bacteria. Deletion of 183 amino acids stretch did not abolished plasmid replication. We have recently cloned both replication modules separately to evaluate which of them supports the observed replication incompatibility of pSmeLPU88b against other plasmids of the collection. The results reported here reveal a widespread compatibility among the conjugal functions of the cryptic plasmids in S. meliloti, and this fact together with the observed high proportion of existing donor genotypes point to the extrachromosomal compartment of the species as being an extremely active plasmid mobilome.