tRNA Genes of Drosophila melanogaster

Abstract

Although the organization of tRNA genes in Drosophila has not been studied as extensively as those of the genes in Xenopus (Birnstiel et al. 1972; Clarkson et al. 1973a,b, 1978; Clarkson and Kurer 1976) and Saccharomyces (Olson et al. 1977; Goodman et al. 1977; Valenzuela et al. 1978; O’Farrell et al. 1978), we are nevertheless beginning to get a much clearer picture of this organization. The reason for this is that in addition to the techniques of genetic analysis, in vitro hybridization, and incorporation of DNA fragments into plasmids, it is possible with Drosophila to localize genes by in situ hybridization of radioactive tRNA to its polytene chromosomes. After autoradiography, tRNA genes may be assigned visually to specific sites on the chromosomes.

THE NUMBER OF tRNA GENES DETERMINED BY HYBRIDIZATION
One of the first questions asked about tRNA genes is how many of them exist in the haploid genome for each tRNA. Ritossa et al. (1966) estimated that there are 750 genes for all species of tRNA by observing the saturation level on hybridizing labeled tRNA to Drosophila DNA. More recently, the hybridization results of Weber and Berger (1976) suggested that Drosophila 4S RNA is composed of approximately 59 families of sequences encoded for by 590 genes. As they pointed out, the proposed number of different sets of tRNA genes is based on several assumptions and, at best, is an average value. The 4S RNA fraction contains not only tRNAs but also fragments of rRNA and other RNAs. Because of...