I-FACTORS IN DROSOPHILA-MELANOGASTER - TRANSPOSITION UNDER CONTROL

I factors are responsible for the I-R system of hybrid dysgenesis in D rosophila melanogaster. They belong to the LINE class of mobile elemen ts, which transpose via reverse transcription of a full-length RNA int ermediate. I factors are active members of the I element family, which also contains defective I elements that are immobilized within peri-c entromeric heterochromatin and represent very old components of the ge nome. Active I factors have recently invaded natural populations of Dr osophila melanogaster, giving rise to inducer strains. Reactive strain s, devoid of active I factors, derive from old laboratory stocks estab lished before the invasion. Transposition of I factors is activated at very high frequencies in the germline of hybrid females issued from c rosses between females from reactive strains and males from inducer st rains. It results in the production of high rates of mutations and chr omosomal rearrangements as well as in a particular syndrome of sterili ty. The frequency of transposition of I factors is dependent on the am ount of full-length RNA that is synthesized from an internal promoter. This full-length RNA serves both as an intermediate of transposition and presumably as a messenger for protein synthesis. Regulators of tra nsposition apparently affect transcription initiation from the interna l promoter. The data presented here lead to the proposal of a tentativ e model for transposition.