Structure and expression of clustered P element homologues in Drosophila subobscura and Drosophila guanche.

Sequence relationships and functional aspects were analysed in the P element homologues of Drosophila subobscura (Ds) and D. guanche (Dg). In both species, the P homologues are clustered at a single genomic position. They lack the characteristic terminal structures of actively transposing P elements, but they have the coding capacity for a 66-kDa 'repressor-like' protein. Two different types of cluster units (G-type and A-type) can be distinguished. The A-type unit, which is present in multiple copies, is transcribed in adult flies. In contrast, the G-type unit has a much lower copy number and is apparently not expressed. In Dg, the isolated G-type sequence carries a 420-bp insertion in the promoter region, which is probably responsible for inactivation. Sequence comparisons of different cluster units show that differentiation of the two types precedes the lineage split of these species. Substitution rates of the deduced proteins reveal two distinct subregions: high variability at the N terminus and strong sequence conservation in the rest of the protein. The variable region contains motifs characteristic of DNA-binding proteins. Adaptive diversification of the cluster units towards specific binding properties might be a plausible explanation for variability in the N-termini. Both unit types have lost the weak promoter region characteristic of P transposons. In the A-type unit, a new promoter has been formed which is apparently composed of parts of insertion sequences derived from two different mobile elements.(ABSTRACT TRUNCATED AT 250 WORDS)