TYPE-I REPRESSORS OF P-ELEMENT MOBILITY

We describe here a family of P elements that we refer to as type I rep ressors. These elements are identified by their repressor functions an d their lack of any deletion within the first two-thirds of the canoni cal P sequence. Elements belonging to this repressor class were isolat ed from P strains and were made in vitro. We found that type I repress or elements could strongly repress both a cytotype-dependent allele an d P element mobility in somatic and germline tissues. These effects we re very dependent on genomic position. Moreover, we observed that an e lement's ability to repress in one assay positively correlated with it s ability to repress in either of the other two assays. The type I fam ily of repressor elements includes both autonomous P elements and thos e lacking exon 3 of the P element. Fine structure deletion mapping sho wed that the minimal 3' boundary of a functional type I element lies b etween nucleotide position 1950 and 1956. None of 12 elements examined with more extreme deletions extending into exon 2 made repressor. We conclude that the type I repressors form a structurally distinct group that does not include more extensively deleted repressor elements suc h as the KP element described previously.