Nucleotide sequence of a complete ribosomal spacer ofD. melanogaster

We determined the nucleotide sequence of a D. melanogaster ribosomal DNA spacer. Sequences of various portions of different cloned ribosomal spacers have been previously reported. We extend the analysis to cover the entire nontranscribed and external transcribed regions. Comparison to other cloned ribosomal DNA gene units of this species confirms a conserved general organization of the ribosomal spacer through different size classes. D. melanogaster ribosomal gene units interrupted by insertions are known to be transcribed at a much lower level than the continuous gene units. Nonetheless previous sequence analysis of a region around the transcription initiation site did not reveal significant differences in rDNA genes with and without insertions. We extend such analysis to cover the last two promoter duplications in the spacer and the entire external transcribed spacer up to the 5' cleavage site of the 18S rRNA.