Plasmid conjugation from Proteobacteria as evidence for the origin of xenologous genes in Cyanobacteria

Comparative genomics showed that 5% of Synechococcus elongatus PCC 7942 genes are of probable proteobacterial origin. To investigate the role of inter-phylum conjugation in cyanobacterial gene acquisition, we tested the ability of a set of prototype proteobacterial conjugative plasmids (RP4, pKM101, R388, R64 and F) to transfer DNA from E. coli to S. elongatus. A series of BioBrick-compatible, mobilizable shuttle vectors was developed. These vectors were based on the putative origin of replication of the Synechococcus resident plasmid pANL. Not only broad-host range plasmids, such as RP4 and R388, but also narrower host-range plasmids, such as pKM101, all encoding MPFT-type IV secretion systems, were able to transfer plasmid DNA from E. coli to S. elongatus by conjugation. Neither MPFF, nor MPFI could be used as interphylum DNA delivery agents. Reciprocally, pANL-derived cointegrates could be introduced in E. coli by electroporation, where they conferred a functional phenotype. These results suggest the existence of potentially ample channels of gene flow between Proteobacteria and Cyanobacteria and point to MPFT-based inter-phylum conjugation as a potential mechanism to explain the proteobacterial origin of a majority of S. elongatus xenologous genes