Transcriptional competition and homeosis in Drosophila.

Interference between different classes of RNA polymerase II alleles causes a mutant phenotype called the "Ubx effect" that resembles one seen in flies haploinsufficient for the transcription factor, Ultrabithorax (Ubx). Flies carrying the mutation in the largest subunit of Drosophila RNA polymerase II, RpII215(4), display the Ubx effect when heterozygous as in RpII215(4)/+ but not when homozygous mutant or wild type. In this report we demonstrate that the interaction between alleles in different classes of polymerase occurs even in the absence of transcription by the wild-type polymerase. We utilized the resistance to the transcriptional inhibitor alpha-amanitin conferred by RpII215(4) to show that RpII215(4)/+ flies raised on alpha-amanitin-containing food still show the Ubx effect and are indistinguishable from flies raised on normal food. We demonstrate using HPLC that the intracellular concentration of alpha-amanitin in the developing larvae is sufficient to inhibit transcription by alpha-amanitin-sensitive polymerase. Furthermore, fluorescein-labeled alpha-amanitin accumulates in imaginal discs, which are the precursor cells for the tissue showing the homeotic transformation in adults. We conclude that the interaction between different classes of RNA polymerase II alleles resulting in the Ubx effect occurs prior to the block in transcription caused by alpha-amanitin.