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

支原体（Molicutes纲）是由一组已知最小的原核生物组成，它们能够在无细胞培养基中生长，没有细胞壁，具有非常小的基因组，G+C含量很低。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。支原体物种的系统发育分析很大程度上依赖于16S rRNA序列。根据16S rRNA序列分析，将支原体属分为5个聚类，即mycoides组、pneumoniae组、hyorhinis组、hominis组和fermentans组。支原体rRNA分子编码基因以操纵子组织，排列顺序为5'-16S-23S-5S-3'，其中单个rRNA基因被内部转录间隔区（ITS）分隔开。支原体只有一个或两个rRNA基因的操纵子。在16S和23S rRNA基因之间的ITS区域已被证明缺乏tRNA基因，并且在序列和长度上随支原体种类的不同而变化。在本研究中，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可以用作确定遗传密切相关物种关系的时计，因为它们比16S rRNA基因具有更高的分化率。我们对脲原体属的研究表明，从其16S rRNA序列和ITS序列得到的系统发育关系几乎相同。

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.