The P-Ph protein-mediated repression of yellow expression depends on different cis- and trans-factors in Drosophila melanogaster

The ph(P1) allele of Drosophila melanogaster encodes a chimeric P-Ph protei n that contains the DNA-binding domain of the P-element transposase and the Ph protein lacking 12 amino-terminal amino acids. it has been shown that t he P-Ph protein is responsible for the formation of a repressive complex on P elements inserted at the yellow locus. Here we demonstrate that an enhan cer element can suppress the P-Ph-mediated inhibition of yellow transcripti on. However, an increase of P-element copy number at the yellow locus overc omes the enhancer effect. The mobilization of P-element transposition induc ed the appearance with a high frequency of Su(y) mutations that partially o r completely suppressed the inhibitory effect of ph(P1) on yellow expressio n. The Su(y) mutations were localized at different sites on chromosomes. On e strong Su(y) mutation, sn(eP1), was found to be induced by a 1.2-kb P-ele ment insertion into the transcribed noncoding region of the singed locus. T he Su(y) mutations resulted in a high level of transcription of the 1.2-kb P element that contained the sequences encoding one DNA-binding and two pro tein-protein interaction domains of the transposase. The effect of Su(y) mu tations can be explained by the competition between the truncated transposa se encoded by a 1.2-kb P element and the P-Ph protein for binding sites on P-element insertions.