Complete sequences of the rRNA genes of Drosophila melanogaster

In this, the first of three papers, we present the sequence of theribosomal RNA (rRNA) genes of Drosophila melanogaster. The gene regions ofD. melanogaster rDNA encode four individual rRNAs: 18S (1,995 nt), 5.8S(123 nt), 2S (30 nt), and 28S (3,945 nt). The ribosomal DNA (rDNA) repeatof D. melanogaster is AT rich (65.9% overall), with the spacers beingparticularly AT rich. Analysis of DNA simplicity reveals that, in contrastto the intergenic spacer (IGS) and the external transcribed spacer (ETS),most of the rRNA gene regions have been refractory to the action ofslippage-like events, with the exception of the 28S rRNA gene expansionsegments. It would seem that the 28S rRNA can accommodate the products ofslippage-like events without loss of activity. In the following two paperswe analyze the effects of sequence divergence on the evolution of (1) the28S gene "expansion segments" and (2) the 28S and 18S rRNA secondarystructures among eukaryotic species, respectively. Our detailed analysesreveal, in addition to unequal crossing-over, (1) the involvement ofslippage and biased mutation in the evolution of the rDNA multigene familyand (2) the molecular coevolution of both expansion segments and thenucleotides involved with compensatory changes required to maintainsecondary structures of RNA.