The host-switching mechanism of phytoplasmas between plants and animals towards the elucidation of the host determination system

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In this year, we determined the complete genomic sequence of the plant-pathogenic bacterium Candidatus Phytoplasma asteris. Comparison of the genome of P.asteris with those of other parasitic and symbiotic bacteria reveals features that are unique to this intracellular parasite, which has adapted to a nutrient-rich environment. The genome of P.asteris encodes genes for basic functions, such as DNA replication, transcription, and translation. In contrast, genes responsible for amino acid and fatty acid biosynthesis, the TCA cycle, and oxidative phosphorylation are absent. This may be because phytoplasmas can depend on their host cells for supplies of many biological substances, as is the case with mycoplasmas. Surprisingly, the P.asteris genome encodes proteins for even fewer metabolic functions than do those of mycoplasmas. For example, the genes for the pentose phosphate cycle and ATP synthase have not been found. R.prowazekii has also lost the genes for the pentose phosphate cycle ; however, this organism has retained genes for the TCA cycle, electron transfer system, and ATP synthase. The complete P.asteris genome suggests that the set of known metabolic pathways of phytoplasmas is even smaller than that of mycoplasmas, representing the most minimal genome among living organisms. Instead, P.asteris possesses genes for transporter proteins involved in the uptake of substances from the surrounding host cell cytoplasm. This may be the result of reductive evolution, as a consequence of life as an intracellular parasite in a nutrient-rich environment Phytoplasmas are unique bacteria that contain a new minimal gene set, and this finding should help to elucidate the host determination mechanisms of phytoplasmas.