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Phytoplasma Replisome Proteins: Diversity, Abundance And Recombination As Potential Tools For Succesful Host Adaptation And Pathogenicity

Phytoplasmas (genus 'Candidatus Phytoplasma') are plant pathogens associated with numerous and diverse plant diseases worldwide. These uncultivated wall-less bacteria inhabiting plant phloem and insect cells, are classified within the class Mollicutes together with acholeplasmas, spiroplasmas and mycoplasmas. In general, phytoplasma genomes are small and reduced, yet repetitive, with often presence of potential mobile units (PMUs). PMUs are transposon-like genetic elements potentially contributing to phytoplasma pathogenicity and their ability for successful colonization of wide range of hosts. Often, they carry putative effector genes as well as replisome genes, such as dnaB, dnaG and ssb that are frequently present on PMUs. In this work, diversity, abundance and possible role of these selected replisome genes in phytoplasma host adaptability and pathogenicity was studied and discussed. Based on our previous work where we sequenced and assembled a draft genome of 'Ca. P. solani' strain SA-1, new primers were designed for amplification of dnaB, dnaG and ssb genes from selected 'Ca. P. solani' isolates. Amplified fragments were sequenced, assembled and analyzed together with sequences retrieved from the Genbank. Phylogenetic analyses of dnaB, dnaG and ssbnucleotide and amino-acid sequences from different phytoplasma species and other members of the class Mollicutes were performed by using MEGA-X software. Our results have shown that all of the three selected replisome genes are present in multiple copies in phytoplasma genomes, particularly SSB proteins that are highly abundant. Phylogenetic analyses have shown clustering and separation of PMUrelated sequences for all three studied replisome genes indicating a complex evolutionary history of phytoplasma replisome proteins. Moreover, our results have indicated frequent intermixing and recombination of PMU genes among distinct phytoplasma species, suggesting that independent and different evolution of non-PMU and PMU replisome sequences could potentially work as a tool enabling better phytoplasma adaptation to different host environments and enhance their pathogenicity.

evolution, phylogeny, phytoplasma, potential mobile unit, replisome

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