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

在这一年，我们确定了植物病原菌Phytoplasma asteris的完整基因组序列。将P.asteris的基因组与其他寄生和共生细菌的基因组进行比较，揭示了这种细胞内寄生虫所特有的特征，它已经适应了营养丰富的环境。紫菀的基因组编码基本功能的基因，如DNA复制，转录和翻译。相反，负责氨基酸和脂肪酸生物合成、TCA循环和氧化磷酸化的基因不存在。这可能是因为植原体可以依赖于它们的宿主细胞来提供许多生物物质，就像支原体一样。令人惊讶的是，紫菀基因组编码的蛋白质比支原体编码的蛋白质的代谢功能更少。例如，尚未发现磷酸戊糖循环和ATP合成酶的基因。普氏罗氏沼虾也丢失了磷酸戊糖循环的基因;然而，这种生物保留了TCA循环、电子转移系统和ATP合成酶的基因。完整的P.asteris基因组表明植原体的已知代谢途径的集合甚至比支原体的更小，代表了活生物体中最小的基因组。相反，P.asteris拥有参与从周围宿主细胞质中摄取物质的转运蛋白基因。这可能是还原进化的结果，作为一个细胞内的寄生虫在一个营养丰富的环境植原体是独特的细菌，含有一个新的最小基因集，这一发现应有助于阐明植原体的宿主决定机制。

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.