Drosophila tRNAVal3b genes have been analyzed with respect to their nucleotide sequence and in vitro transcription efficiency. Plasmid pDt78R contains a single tRNA gene derived from the major tRNAVal3b gene cluster at chromosome band 84D. Its sequence corresponds to that of the tRNAVal3b. Two other plasmids, pDt41R and pDt48, each contain a tRNAVal3b-like gene from the minor tRNAVal3b gene cluster at chromosome bands 90BC. They contain the expected CAC anticodon, but their sequence differs from the tRNA at four positions. In homologous cell-free extracts, the tRNAVal3b variant genes in pDt41R and pDt48 are transcribed an order of magnitude more efficiently than the tRNAVal3b gene in pDt78R. However, the variant genes do not appear to contribute significantly to the in vivo tRNA pool [Larsen et al.: Mol. Gen. Genet. 185 (1982) 390-396]. We propose the term allogenes to describe families of related DNA sequences that may code for variant forms of a standard tRNA isoaccepting species.
