On the redundancy of DNA complementary to amino acid tranfer RNA and its absence from the nucleolar organizer region of Drosophila melanogaster.

MOLECULAR hybridization (HALL and SPIEGELMAN 1961 ) of labeled ribosomal RNA (r-RNA) with DNA in D. mlanogaster has revealed (VERMEULEN and ATWOOD 1965; RITOSSA and SPIEGELMAN 1965) that, as in the bacteria (YANKOFSKY and SPIEGELMAN 1962a, b, 1963), approximately 0.27% of the DNA is complementary to r-RNA. This DNA is believed to constitute the ensemble of templates for the transcription of r-RNA and may be referred to as the r-DNA. The amount of r-DNA per haploid Drosophila genome is sufficient to complement at least 130 molecules each of 18s and 28s r-RNA. Inversions in the X chromosome of Drosophila melanoguster are available (SIDOROV 1930; STURTEVANT and BEADLE 1936; MULLER et al. 1937) from which one can derive X chromosomes possessing duplications or deletions of a heterochromatic region which includes the nucleolus organizer (NO). With these chromosomes, flies can be obtained which have from one to four doses of the NO region. With DNA from such flies, annealing experiments demonstrated ( RITOSSA and SPIEGELMAN 1965) that the amount of r-RNA hybridizable per unit of DNA was directly proportional to the dosage of the NO region per genome. These data indicated then that the DNA sequences complementary to the ribosomal RNA are confined to the segment contained in the deletion or duplication employed. The existence in bacteria of DNA complementary to amino acid transfer RNA (t-RNA) has been established ( GIACOMONI and SPIEGELMAN 1962; GOODMAN and RICH 1962). However, this issue has thus far not been taken up in the higher forms. It is, in principle, readily resolvable by molecular hybridization with suitably labeled and purified t-RNA. The D. melanogaster DNA used in the studies on ribosomal RNA permits the performance of the necessary experiments as well as others relevant to the following two questions: (1 ) What proportion of the D. melanogaster genome is complementary to t-RNA? (2) Are the DNA complements of t-RNA localized in the same region as those of the r-RNA? An answer to the first question is pertinent to possible interpretations of the 130-fold redundancy in ribosomal DNA complements. The second question is of obvious interest and gains particular importance from recent reports of t-RNA ' The In\estigations reported here were supported by Grants GB-2169, GB-2700, GB-296 from the National Science Foundation, and by Public Health Service Research Grant No CA-01094 from the National Cancer Institute On leave from the International Laboratory of Genetics and Biophysics