Conjugal properties of the Sinorhizobium meliloti plasmid mobilome

Abstract

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 N(2)-fixing legume-symbiont Sinorhizobium meliloti. 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 operational taxonomic units (OTUs)]. The cumulative Shannon index calculated for the observed plasmid profiles showed a clear saturation plateau, 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 to a third recipient strain. 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 c. 20% of the OTUs. 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, points to the extrachromosomal compartment of the species as being an extremely active plasmid mobilome.

The complete genome sequence of the dominant Sinorhizobium meliloti field isolate SM11 extends the S. meliloti pan-genome

“…The fact that additional plasmid survival genes and relicts thereof were identified in pSmeSM11c supports this assumption. Importance of the S. meliloti mobilome (collectivity of mobile plasmids within this species) has been outlined in recent publications (Kuhn et al, 2008;Pistorio et al, 2008). Presence of genes on pSmeSM11c that are highly similar to corresponding genes of other rhizobial species indicates that there is vivid exchange of genetic information within the Rhizobiaceae.…”

“…In contrast to S. meliloti Rm1021, many S. meliloti field isolates harbour additional, so-called accessory plasmids specifying part of the mobilome of this species (Kuhn et al, 2008;Pistorio et al, 2008). Mobile plasmids contribute to exchange of genetic information among the population and are likely to be involved in gene cluster translocation events within the genome.…”

Characterization of a collection of plasmid-containing bacteria isolated from an on-farm biopurification system used for pesticide removal

“…In order to select those isolates that harbored one or more plasmids, the in situ lysis gel assay described by Eckhardt (1978) was used with the modifications described by Pistorio et al (2008).…”

Rhizobial communities in symbiosis with legumes: genetic diversity, competition and interactions with host plants

“…This distinction between the rhizobial chromosome and plasmids in the context of `evolutionary plasticity' and the role of these replicons in bacterial diversification resulted in the emergence of genomical concepts, which are similar to the pan-genome idea. The terms `pan-plasmidome' (pool of plasmids of a species or genus) or `mobilome' (pool of transferable plasmids of a species) were introduced [44,45], and these concepts of rhizobial genomics are also taken into consideration with reference to the diversity and evolution of legume microsymbionts.…”