INSTITUTO DE BIOTECNOLOGIA Y BIOLOGIA MOLECULAR
Unidad Ejecutora - UE
congresos y reuniones científicas
THE CRYPTIC-PLASMID MOBILOME IN THE N2-FIXING SOIL-BACTERIA Sinorhizobium meliloti.
GIUSTI, M. DE LOS A.; TORRES TEJERIZO, G. A.; DEL PAPA, M.F.; LOZANO, M.J.; DRAGHI, W. O.; PISTORIO, M.; LAGARES, A.
Conferencia; INTERNATIONAL PLASMID BIOLOGY CONFERENCE 2008; 2008
International Society for Plasmid Biology
The biology and biochemistry of plasmid transfer in soil bacteria is currently under active investigation becauseof its central role in prokaryote adaptation and evolution. In this work, we examined the conjugal properties ofthe cryptic plasmids present in a collection of the N2-fixing legume-symbiont Sinorhizobium meliloti in order toinvestigate how frequently conjugative/mobilizable plasmids are found in the collected diversity within the species,and the degree of cross-complementation among the helper functions. The study was performed on 65 S. melilotiisolates recovered from 25 humic soils of Argentina, which were grouped into 22 plasmid-profile types (i.e., plasmidOTUs-Operational Taxonomic Units). The cumulative Shannon index calculated for the observed plasmid profilesshowed a clear saturation plateau for n>49 (n = number of isolates used for the calculation), thus indicating anadequate representation of the S. meliloti plasmid-profile types in the isolates studied. The results show thatisolates of nearly 14% of the plasmid OTUs hosted transmissible plasmids and that isolates of 29% of the plasmidOTUs were able to retransfer the previously characterized mobilizable-cryptic plasmid pSmeLPU88b (Pistorio etal., 2003) to a third recipient strain. The proportion of rhizobia carrying transmissible replicons was inferred fromexperiments in the laboratory. It is noteworthy that isolates belonging to 14% of the plasmid OTUs proved to berefractory to the entrance of the model plasmid pSmeLPU88b, suggesting either the presence of surface exclusionphenomena or the occurrence of restriction incompatibility with the incoming replicon. Incompatibility for replicationbetween resident plasmids and plasmid pSmeLPU88b was observed in ca. 20% of the OTUs. We have previouslyreported that plasmid pSmeLPU88b carries two functional plasmid replication modules, one of them belonging to theRepABC-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 endcompared 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 supportsthe observed replication incompatibility of pSmeLPU88b against other plasmids of the collection.The results reportedhere reveal a widespread compatibility among the conjugal functions of the cryptic plasmids in S. meliloti, andthis fact together with the observed high proportion of existing donor genotypes point to the extrachromosomalcompartment of the species as being an extremely active plasmid mobilome.