Apparent lack of discrimination in the reading of certain codons in Mycoplasma mycoides.

We report a cluster of four tRNA genes from Mycoplasma mycoides as well as the sequence of the alanine, proline, and valine tRNAs and the serine tRNA reading the UCN codons (where N stands for G, A, C, or U). This brings the total number of tRNA genes that we have so far characterized in this organism to 14, 6 of which code for tRNAs that read the codons of family boxes. In each of these latter cases, we found only one gene per family box, and the gene sequence contains a thymidine in the position corresponding to the wobble nucleotide, with the exception of the arginine tRNA gene that has an adenosine in this position. Furthermore, all of the tRNA structures reported here have an unsubstituted uridine in the wobble position. These findings are similar to those reported for mitochondria, especially yeast mitochondria, that contain an arginine tRNA with the anticodon ACG. However, the resemblance is not complete since we have demonstrated the presence of two isoacceptor tRNAs for threonine having uridine and adenosine, respectively, in the wobble position. It is suggested that in the M. mycoides at least some of the family codon boxes are read by only one tRNA each, using an unconventional method without discrimination between the nucleotides in the third codon position.