Restriction map of 5S RNA genes of Drosophila melanogaster

Abstract

ALTHOUGH certain segments of the primary sequence and secondary structure of 5S RNA seem to be strikingly similar in prokaryotes and eukaryotes¹, the genetic organisation of the 5S RNA genes themselves exhibits remarkable variability. In E. coli, several lines of evidence indicate that the multiple copies of the 5S, 16S and 23S rRNA genes are organised into transcription units containing one gene each plus a sequence coding either for tRNA₁^ILE or tRNA₂^GLU such that they are cotranscribed in the order 16S–4S–23S–5S RNA (refs 2 and 3). In eukaryotes, the 5S RNA genes may be clustered at a single site as in humans^4,5 or widely distributed on many chromosomes as in the case of Xenopus laevis, where the 5S genes are found on the telomeres of most, if not all, of the 18 chromosomes⁶. Among the eukaryotes, there is no example in which the 5S genes are tightly linked to the 18S and 28S rRNA genes. Although there has been a large number of papers reporting the cytogenetic localisation of the 5S RNA genes in various organisms, only in the case of Xenopus laevis are there data concerning the physical organisation of these genes⁷. Even in this instance, however, the structure is only known for a few 5S gene repeat lengths, and the long range order that might be imposed on an entire 5S DNA cluster within the chromosome remains obscure. Whereas long range order in certain satellite sequences has been described^8,9, similar higher order organisation has not as yet been revealed for transcribed redundant genes.