Analysis of a Drosophila tRNA gene cluster: Two tRNA Leu genes contain intervening sequences

Abstract

A recombinant DNA phage containing a cluster of Drosophila melanogaster tRNA genes has been isolated and analyzed. The insert of this phage has been mapped by in situ hybridization to chromosomal region 50AB, a known tRNA site. Nucleotide sequencing of the entire Drosophila tRNA coding region reveals seven tRNA genes spanning 2.5 kb of chromosomal DNA. This cluster is separated from other tRNA regions on the chromosome by at least 2.7 kb on one side, and 9.6 kb on the other. Two tRNA genes are nearly identical and contain intervening sequences of length 38 and 45 bases, respectively, in the anticodon loop. These two genes are assigned to be tRNA Leu genes because of significant sequence homology with yeast tRNA 3 Leu, and secondary structure homology with yeast tRNA 3 Leu intervening sequences. In addition, an 8 base sequence (AAAAUCUU) is conserved in the same location in the intervening sequences of Drosophila tRNA Leu genes and a yeast tRNA 3 Leu gene. Similar sequences occur in all other tRNAs containing intervening sequences. The remaining five genes are identical tRNA Ile genes, which are also identical to a tRNA Ile gene from chromosomal region 42A. The 5′ flanking regions are only weakly homologous, but each set of isoacceptors contains short regions of strong homology approximately 20 nucleotides preceding the tRNA coding sequences: GCNTTTTG preceding tRNA Ile genes; and GANTTTGG preceding tRNA Leu genes. The genes are irregularly distributed on both DNA strands; spacing regions are divergent in sequence and length.