Role of natural transformation in the evolution of small cryptic plasmids in Synechocystis sp. PCC 6803

Abstract

Small cryptic plasmids have no clear effect on the host fitness and theirfunctional repertoire remains obscure. The naturally competent cyanobacte-riumSynechocystissp. PCC 6803 harbours several small cryptic plasmids;whether their evolution with this species is supported by horizontal transferremains understudied. Here, we show that the small cryptic plasmid DNA istransferred in the population exclusively by natural transformation, wherethe transfer frequency of plasmid-encoded genes is similar to that ofchromosome-encoded genes. Establishing a system to follow gene transfer,we compared the transfer frequency of genes encoded in cryptic plasmidspCA2.4 (2378 bp) and pCB2.4 (2345 bp) within and between populations oftwoSynechocystissp. PCC 6803 labtypes (termed Kiel and Sevilla). Ourresults reveal that plasmid gene transfer frequency depends on the recipientlabtype. Furthermore, gene transfer via whole plasmid uptake in the Sevillalabtype ranged among the lowest detected transfer rates in our experi-ments. Our study indicates that horizontal DNA transfer via natural transfor-mation is frequent in the evolution of small cryptic plasmids that reside innaturally competent organisms. Furthermore, we suggest that the contribu-tion of natural transformation to cryptic plasmid persistence inSynechocys-tisis limited.