Sequence organization of the ribosomal spacer of D.melanogaster

We subcloned several fragments of the rDNA nontranscribed spacer (NTS) of D.melanogaster and analysed their nucleotide sequence. The central portion contains a variable number of 240 base pair repeated sequences ending with two contiguous Alu I sites. Within every repeat there is a 42 base pair sequence perfectly homologous to the transcription initiation site from -18 to +24. Sequences homologous to the basic 240 bp repeat are present both at the end of the unique 1900 bp region downstream to the transcription termination site and within the unique region preceding the transcription initiation site up to nucleotide -140. These results are discussed in relation to the general problem of ribosomal gene units and their evolution.