Comparative genome analysis of Candidatus Phytoplasma australiense (subgroup tuf-Australia I; rp-A) and Ca. Phytoplasma asteris strains OY-M and AY-WB

The chromosome sequence of Candidatus Phytoplasma australiense (subgroup tuf-Australia I; rp-A), associated with dieback in papaya, Australian grapevine yellows in grapevine, and several other important plant diseases, was determined. The circular chromosome is represented by 879,324 nucleotides, a GC content of 27%, and 839 protein-coding genes. Five hundred two of these protein-coding genes were functionally assigned, while 337 genes were hypothetical proteins with unknown function. Potential mobile units (PMUs) containing clusters of DNA repeats comprised 12.1% of the genome. These PMUs encoded genes involved in DNA replication, repair, and recombination; nucleotide transport and metabolism; translation; and ribosomal structure. Elements with similarities to phage integrases found in these mobile units were difficult to classify, as they were similar to both insertion sequences and bacteriophages. Comparative analysis of Ca. Phytoplasma australiense with Ca. Phytoplasma asteris strains OY-M and AY-WB showed that the gene order was more conserved between the closely related Ca. Phytoplasma asteris strains than to Ca. Phytoplasma australiense. Differences observed between Ca. Phytoplasma australiense and Ca. Phytoplasma asteris strains included the chromosome size (18,693 bp larger than OY-M), a larger number of genes with assigned function, and hypothetical proteins with unknown function.