Tsetse fly rDNA: an analysis of structure and sequence.

A genomic library of Glossina morsitans morsitans (tsetse fly) has been constructed in the phage vector EMBL 4 and a complete rDNA unit isolated by using a D. melanogaster rDNA clone as a probe. The overall organisation is typical of higher eukaryotes, including an intergenic spacer consisting of a subrepeating structure. Atypically, however, the 45S precursor RNA promoter was shown to lie within the last subrepeat by S1 mapping; i.e. the last subrepeat extends 90 bp into the ETS. The sequence of the spacer subrepeats, the ETS and the first 151 nucleotides of the 18S gene was determined. Comparisons with the corresponding regions of other higher eukaryotes, including insects shows that the ETS has completely diverged, raising questions concerning their functional significance and evolutionary retention; depending on the method of alignment, only two short regions of reasonable homology are shared with Drosophila species: a stretch of nucleotides around the transcription initiation site, and AACATA at the NTS-18S gene junction; and the functionally important G at -16, conserved in all other examined species, is displaced no matter what method of alignment is used. These and other features reflect continual processes of change in the rDNA family to which the several functions of the repeating unit need to adjust.