EVOLUTION OF TRANSFER-RNAS AND TRANSFER-RNA GENES IN ACHOLEPLASMA-LAIDLAWII

The genes for 22 tRNA species from Acholeplasma laidlawii, belonging to the class Mollicutes (Mycoplasmas), have been cloned and sequenced. Sixteen genes are organized in 3 clusters consisting of eleven, three and two tRNA genes, respectively, and the other 6 genes exist as a single gene. The arrangement of tRNA genes in the 11-gene, the 3-gene and the 2-gene clusters reveals extensive similarity to several parts of the 21-tRNA or 16-tRNA gene cluster in Bacillus subtilis. The 11-gene cluster is also similar to the tRNA gene clusters found in other mycoplasma species, the 9-tRNA gene cluster in M. capricolum and in M. mycoides, and the 10-tRNA gene cluster in Spiroplasma meliferm. The results suggest that the tRNA genes in mycoplasmas have evolved from large tRNA gene clusters in the ancestral Gram-positive bacterial genome common to mycoplasmas and B. subtilis. The anticodon sequences including base modifications of 15 tRNA species from A. laidlawii were determined. The anticodon composition and codon-recognition patterns of A. laidlawii resemble those of Bacillus subtilis rather than those of other mycoplasma species.