Repressor of P elements in Drosophila melanogaster: Cytotype determination by a defective P element carrying only open reading frames 0 through 2.

The P element is a type of transposable element in Drosophila melanogaster. Characteristics of the syndrome of "hybrid dysgenesis" are due to transposition of P elements, and the molecular mechanism for regulation of this transposition has been unknown. In this study a Q strain (which carries only defective P elements in its genome but still is able to repress the transposition of complete P elements although defective in transposase activity) was used to determine the structure of the P element with this repressor (or P cytotype-determining) domain. Examination of the cytotype and structure of the P elements of particular strains with reduced copy number of P elements showed that the P element with a repressor domain was defective, being deleted between bases 1991 and 2448. This region corresponds to most of the third intron [between open reading frame (ORF) 2 and ORF 3] as well as half the ORF 3 of an intact P element. Therefore ORF 3 was deemed to be unnecessary for repressor production.