Five transfer RNA genes lacking CCA termini are clustered in the chromosome of Streptomyces rimosus.

The nucleotide sequence of a 1105 bp Streptomyces rimosus DNA fragment containing five transfer RNA genes was determined. Two tRNA(Gln) (CUG) genes, differing by 1 bp in the aminoacyl stem, and three identical tRNA(Glu) (CUC) genes were identified. The five tRNA genes, arranged in the order: Gln1-Glu1-Glu2-Gln2-Glu3, were separated by short, nonhomologous intergenic regions. Surprisingly, none of these tRNA genes encoded the CCA 3' terminus of mature tRNAs. All five encoded tRNAs for the translation of GC rich codons, which are preferentially used in Streptomyces genes (CAG and GAG, respectively). We recently reported nucleotide sequences of two initiator tRNA genes from S. rimosus, which also do not encode the CCA end of mature tRNAs. It is therefore very likely that S. rimosus represents an example of those eubacteria in which the majority of tRNA genes do not encode the 3' terminal CCA end of mature tRNAs. Evolutionary implications of this finding remain to be elucidated.