Trans-inactivation of the Drosophila brown gene: evidence for transcriptional repression and somatic pairing dependence.

Position-effect variegation in Drosophila is the variable inactivation of a gene that occurs when it is juxtaposed to heterochromatic regions of chromosomes. The brown gene, required for pteridine pigment in the eye, is unusual in that expression of the unrearranged homolog also is affected. This dominant effect can be very strong, as inactivation is detectable when as many as three trans copies of the gene are present. We show that pteridine reductions coincide with similar reductions in the accumulation of brown mRNA. The dominant effect is suppressed by certain altered structural configurations of the brown region, suggesting that somatic pairing is involved in the phenomenon. We propose that direct transmission of the altered chromatin structure characteristic of position-effect variegation (heterochromatinization) occurs between paired homologs in the region of the brown locus.