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Serine tRNAs and their genes in Drosophila melanogaster Cribbs, David Lamar


Serine tRNAs and their genes in Drosophila melanogaster were characterized. The nucleotide sequences of tRNA₄[sup=Ser] (codon UCG), tRNA₇[sup=Ser] (UCA, UCC, UCU), and tRNA[sub=2b][sup=Ser] (AGC, AGU) were determined. Also, the nucleotide sequences of four tRNA[sup=Ser] genes from the X chromosome isolated in recombinant plasmids were determined. Transfer RNA₄[sup=Ser] and tRNA₇[sup=Ser] differ at only three out of 85 positions, including the "wobble" nucleotide of the anticodon. However, tRNA[sub=2b][sup=Ser] is only 72% homologous with tRNA₄[sup=Ser] (62 out of 85 positions, not counting differences in modification). Major regions of sequence homology (> 5 consecutive positions) are found only in the D arm (21 consecutive positions) and in the TѱC arm (11 consecutive positions). Transfer RNA₄[sup=Ser] and tRNA₇[sup=Ser] are indistinguishable by RNA-DNA hybridization. Both hybridize to the same sites on polytene chromosomes in situ, including the major site at 12DE on the X chromosome, and 23E on chromosome 2 (Hayashi et al. (1980). Chromosoma 76, 65-84.) No other purified tRNA than tRNAs[sub=4,7][sup=Ser] has been shown to hybridize to the X chromosome in Drosophila. Therefore, several X-derived recombinant plasmids hybridizing tRNA[sub=4,7][sup=Ser] (pDt 16, pDt 17R, pDt 27R, and pDt 73; Dunn et al. (1979). Gene ], 197-215.) were analyzed. Based on the results of Southern blotting experi-ments, there appear to be eight tRNA[sup=Ser] genes on the four plasmids (one each on pDt 17R and pDt 73; two on pDt 16; and four on pDt 27R). Thus, the 12DE region contains at least eight tRNA[sup=Ser] genes. Restriction mapping and DNA sequence analysis were performed with pDt 16, pDt 17R, and pDt 73. Based on the tRNA sequences, which differ at three positions, the presumptive DNA sequences encoding tRNA[sub=4] [sup=Ser] and tRNA[sub=7] [sup=Ser] can be represented as 444 or 777 genes. DNA sequence analysis gave surprising Ser results in this respect. Analysis of four tRNA[sup=Ser] genes on the three plasmids S identified two 777 genes matching tRNA₇[sup=Ser] plus "hybrid" 774 and 474 sequences. Further, pDt 16 contains both a 777 and a 774 gene as a direct repeat 400 Ser base pairs apart. Since a 444 gene corresponding to tRNA₄[sup=Ser] must exist, there are at least four different types of closely related serine tRNA genes in the D. melanogaster genome. This observation may have implications concerning the evolution and maintenance of reiterated tRNA genes in eukaryotes. In addition to studies on serine tRNAs and their genes, the nucleotide sequences of Drosophila tRNA₅[sup=Lys] and of a tRNA[sup=Arg] were determined.

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