Conservation of the unique rickettsial rRNA gene arrangement in Anaplasma.

The organization of the rRNA genes of Anaplasma marginale, the type species of the genus Anaplasma, was identified to determine if the atypical rRNA gene arrangement identified in rickettsiae preceded divergence of the order Rickettsiales into the families Anaplasmataceae and Rickettsiaceae. The rRNA genes are encoded by two unlinked units, each present in a single copy per A. marginale genome. The 16S rRNA gene is separated from the linked 23S and 5S rRNA genes by a minimum of 100 kb. Similar to species belonging to the genus Rickettsia, the typical bacterial 16S-23S spacer region containing tRNA genes has been lost in A. marginale. In contrast, the fmt gene located upstream of the 23S rRNA gene in most Rickettsia spp. is not maintained in A. marginale, consistent with the fmt arrangement being a relatively late event in the evolution of rickettsial species.