POSITION-EFFECT VARIEGATION IN DROSOPHILA DEPENDS ON THE DOSE OF THE GENE ENCODING THE E2F TRANSCRIPTIONAL ACTIVATOR AND CELL-CYCLE REGULATOR

A dominant mutation due to the insertion of a P-element at 93E on the third chromosome of Drosophila melanogaster enhances position-effect v ariegation. The corresponding gene was cloned by transposon tagging an d the sequence of the transcript revealed that it corresponds to the g ene encoding the transcriptional activator and cell cycle regulator dE 2F. The transposon-tagged allele is homozygous viable, and the inserti on of the transposon in an intron correlates with a strong reduction i n the amount of transcript. A homozygous lethal null allele was found to behave as a strong enhancer when heterozygous. Overexpression of th e gene in transgenic flies has the opposite effect of suppressing vari egation. A link is established here, and discussed, between the dose o f a transcriptional activator, which controls the cell cycle, and epig enetic silencing of chromosomal domains in Drosophila.