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We have determined the DNA sequence of a cloned cluster of essentially identical glutamate tRNA genes of D. melanogaster. The cluster consists of five genes: a gene triplet spanning approximately 0.55 kb followed by a 0.45 kb gene doublet 3.0 kb downstream. The genes are all arranged with the same polarity, do not encode the tRNA CCA end and contain no intervening sequences. Examination of the 5' and 3' sequences immediately flanking each gene reveals a striking pattern of sequence homologies between certain of the genes, which suggests a possible evolutionary history of this gene cluster. We propose that two ancestral genes each gave rise to gene doublets by duplication, while one of these gene pairs then gave rise, in turn, to a trio of genes as a result of unequal crossover.

We have isolated four segments of Drosophila melanogaster DNA that hybridize to homologous initiator tRNAMet. Three of the cloned fragments contain initiator tRNA genes, each of which can be transcribed in vitro. The fourth clone, pPW568, contains an initiator tRNA pseudogene which is not transcribed in vitro by RNA polymerase III. The pseudogene is contained in a 1.15 kb DNA fragment. This fragment has the characteristics of dispersed repetitive DNA and hybridizes in situ to at least 30 sites in the Drosophila genome. The arrangement of the initiator tRNA genes we have isolated, is different to that of other Drosophila tRNA gene families. The initiator tRNA genes are not clustered nor intermingled with other tRNA genes. They occur as single copies within an approximately 415-bp repeat segment, which is separated from other initiator tRNA genes by a mean distance of 17 kb. In situ hybridization to polytene chromosomes localizes these genes to the 61D region of the Drosophila genome. Hybridization analysis of genomic DNA indicates the presence of 8-9 non-allelic initiator tRNA genes in Drosophila melanogaster.