Phytoplasmas are insect-transmitted, uncultivable bacterial plant pathogens that cause diseases in hundreds of economically important plants. They represent a monophyletic group within the class Mollicutes (trivial name mycoplasmas) and are characterized by a small genome with a low GC content, and the lack of a firm cell wall. All mycoplasmas, including strains of 'Candidatus (Ca.) Phytoplasma asteris' and 'Ca. P. australiense', examined so far have circular chromosomes, as is the case for almost all walled bacteria.
Our work has shown that 'Ca. Phytoplasma mali', the causative agent of apple proliferation disease, has a linear chromosome. Linear chromosomes were also identified in the closely related provisional species 'Ca. P. pyri' and 'Ca. P. prunorum'. The chromosome of 'Ca. P. mali' strain AT is 601,943 bp in size and has a GC content of 21.4%. The chromosome is further characterized by large terminal inverted repeats and covalently closed hairpin ends. Analysis of the protein-coding genes revealed that glycolysis, the major energy-yielding pathway supposed for 'Ca. P. asteris', is incomplete in 'Ca. P. mali'. Due to the apparent lack of other metabolic pathways present in mycoplasmas, it is proposed that maltose and malate are utilized as carbon and energy sources. However, complete ATP-yielding pathways were not identified. 'Ca. P. mali' also differs from 'Ca. P. asteris' by a smaller genome, a lower GC content, a lower number of paralogous genes, fewer insertions of potential mobile DNA elements, and a strongly reduced number of ABC transporters for amino acids. In contrast, 'Ca. P. mali' has an extended set of genes for homologous recombination, excision repair and SOS response than 'Ca. P. asteris'.
The small linear chromosome with large terminal inverted repeats and covalently closed hairpin ends, the extremely low GC content and the limited metabolic capabilities reflect unique features of 'Ca. P. mali', not only within phytoplasmas, but all mycoplasmas. It is expected that the genome information obtained here will contribute to a better understanding of the reduced metabolism of phytoplasmas, their fastidious nutrition requirements that prevented axenic cultivation, and the mechanisms involved in pathogenicity.