Analysis and application of the 16S-23S rRNA intergenic spacers in mycoplasmas

支原体是由已知的最小的原核生物组成的一组，能够在无细胞的介质中生长，没有细胞壁，具有非常小的G+C含量的基因组。支原体属的成员在自然界中无处不在，它们的宿主从植物到动物都有。支原体的系统发育分析在很大程度上依赖于16S rRNA序列。根据16S rRNA序列分析，将支原体分为5个类群：支原体群、肺炎支原体群、猪鼻支原体群、人型支原体群和发酵支原体群。支原体rRNA分子编码基因按5‘-16S-23S-5S-3’顺序排列在操纵子中，其中单个rRNA基因由内部转录间隔区(ITS)分隔。支原体对rRNA基因只有一个或两个操纵子。16S和23S rRNA基因之间的ITS区缺乏tRNA基因，其序列和长度因支原体种类而异。在这项研究中，我们对支原体16S和23S rRNA基因之间的ITS区域进行了测序和分析，作为16S rRNA的替代，因为最近的研究表明，ITS可以作为计时器来确定遗传关系密切的物种的亲缘关系，因为它们比16S rRNA基因有更高的差异率。我们以前对解脲原体属的研究表明，16S rRNA和ITS序列的系统发育关系几乎是相同的。

Mycoplasmas (class Molicutes) are consisting of group of the smallest prokaryotes known that are able to grow in cell-free media, lack a cell wall, and have very small genomes with a low G+C content. Members of the genus Mycoplasma are ubiquitous in nature, and their hosts range from plants to animals. The phylogenetic analyzes of the Mycoplasma species largely depends on the 16S rRNA sequence. On the basis of the 16S rRNA sequence anlysis, the genus Mycoplasma is divided into five clsters, M.mycoides group, M.pneumoniae group, M.hyorhinis group, M.hominis group, and M.fermentans group. The genes coding for rRNA molecules of mycoplasmas are organized in operons and arranged in the order of 5'-16S-23S-5S-3', in which the individual rRNA genes are separated by internal transcribed spacer (ITS) regions. Mycoplasmas have only one or two operons for the rRNA genes. The ITS regions between the 16S and 23S rRNA genes have been shown to lack tRNA genes and to be variable in seqence and length depending on the Mycoplasma species. In this study we sequenced and analyzed the mycoplasmal ITS region between the 16S and 23S rRNA genes as an alternative to 16S rRNA,because recent studies indicate the ITS can be used as chronometers for determining the relationships of genetically closely related species, as they have higher rates of divergence than 16S rRNA genes. Our previous studies on the genus Ureaplasma indicated that the phylogenetic relationships derived from the 16S rRNA and ITS sequences are almost identical.