Phylogenetic analyses of phytoplasma replisome proteins demonstrate their distinct and complex evolutionary history

The genus ‘Candidatus Phytoplasma’ encompasses diverse groups of endocellular bacteria without cell wall that reside within plant phloem and insect cells. They are classified to the class Mollicutes together with acholeplasmas, spiroplasmas, mycoplasmas and other related bacteria. One of main characteristics is their small and reduced genomes with the often presence of potential mobile units (PMUs). PMUs are transposon-like genetic elements suggested to contribute to the ability of these pathogens in colonization of wide range of hosts. Replisome genes, such as dnaB, dnaG and SSB are frequently found in PMUs. In this study, their possible role in independent PMU replication is investigated and discussed. Herein, the extensive phylogenetic analysis of DnaB, DnaG and SSB replisome PMU sequences of different members of the class Mollicutes was performed. The results indicate a complex evolutionary history of phytoplasma PMU replisome proteins. For DnaB sequences, phylogenetic analysis revealed separation into DnaB1 and DnaB2 clusters with additional clustering of PMU and non-PMU sequences within DnaB1. Separate grouping of the PMU sequences was also shown for DnaG and SSB sequences. These results suggest distinct and independent evolution of non-PMU and PMU replisome sequences and frequent intermixing and recombination of PMU genes among different phytoplasma species enabling their better adaptation to different environment.