Construction of the mycoplasma ev sequence data (acholeplasma/anaeroplasma/spiroplasma/ureaplasma)

The 5S rRNA sequences of eubacteria and mycoplasmas have been analyzed and a phylogenetic tree con- structed. We determined the sequences of 5S rRNA from Clos- tridium innocuum, Acholeplasma laidlawii, Acholeplasma mod- icum, Anaeroplasma bactoclasticum, Anaeroplasma abactoclas- ticum, Ureaplasma urealyticum, Mycoplasma mycoides mycoides, Mycoplasma pneumoniae, and Mycoplasma gallisep- ticum. Analysis of these and published sequences shows that mycoplasmas form a coherent phylogenetic group that, with C. innocuum, arose as a branch of the low G+C Gram-positive tree, near the lactobacilli and streptococci. The initial event in mycoplasma phylogeny was formation of the Acholeplasma branch; hence, loss of cell wall probably occurred at the time of genome reduction to =1000 MDa. A subsequent branch produced the Spiroplasma. This branch appears to have been the origin of sterol-requiring mycoplasmas. During develop- ment of the Spiroplasma branch there were several indepen- dent genome reductions, each to =500 MDa, resulting in My- coplasma and Ureaplasma species. Mycoplasmas, particularly species with the smallest genomes, have high mutation rates, suggesting that they are in a state of rapid evolution.