A pairing-sensitive element that mediates trans-inactivation is associated with the Drosophila brown gene.

Abstract

Position-effect variegation in Drosophila is the mosaic expression of a gene juxtaposed to heterochromatin by chromosome rearrangement. The brown (bw +) gene is unusual in that variegating mutations are dominant, causing "trans-inactivation" of the homologous allele. We show that copies of bw + transposed to ectopic sites are not trans-inactivated by rearrangements affecting the endogenous gene. However, when position-effect variegation is induced on an ectopic copy by chromosome rearrangement, the allele on its paired homolog is trans-inactivated, whereas other copies of bw + are not. This confirms that trans-inactivation is "chromosome local" and maps the responsive element to the immediate vicinity of brown. Subsequent P-transposase-induced deletions within the ectopic copy in cis to the rearrangement breakpoint caused partial suppression of trans-inactivation. Surprisingly, the amount of suppression was correlated with deletion size, with some degree of trans-inactivation persisting even when the P[bw +] transposon was completely excised. The chromosome-local nature of the phenomenon and its extreme sensitivity to small disruptions of somatic pairing leads to a model in which a regulator of the brown gene is inactivated by direct contact with heterochromatic proteins.[Key Words: Position-effect variegation; Drosophila; somatic pairing; in vivo deletion mappingl Received November 21, 1990; accepted December 18, 1990.A common class of mutations induced by ionizing radiation in Drosophila are the variegating position effects. These are generally caused by chromosomal rearrangements placing euchromatic genes in the vicinity of heterochromatin, the compacted regions of chromosomes that remain condensed during most of the cell cycle (for review, see Spofford 1976; Henikoff 1990). The variegated phenotype is caused by reduction in gene expression in some cells but not in others, evidently as a result of transcriptional inactivation (Henikoff 1981;Rushlow et al. 1984;Henikoff and Dreesen 1989). Consistent with this interpretation, position-effect variegation of nearly all genes is recessive, implying that inactivation of one copy of a gene in cis has no effect on the copy in trans. However, variegated position effects on a few genes, including the brown (bw +) gene, are dominant (Muller 1932;Slatis 1955;Stem and Kodani 1955;Henikoff 1979;O'Donnell et al. 1989).Recently, we showed that dominant position-effect variegation of brown involves a sharp reduction in mRNA accumulation from both copies of the gene, the copy in cis to the rearrangement breakpoint (cis-inactivation) and the copy in trans (trans-inactivation). We proposed a somatic pairing model in which the brown gene region, inactivated by virtue of its juxtaposition to heterochromatin, could transmit the inactivated state to the copy of brown in trans. Evidence for this model was the sensitivity of trans-inactivation to chromosome configurations that are expected to disrupt somatic pairing in the vicinity of the brown gene.The somatic pairing model makes sev...

Intergenic transcription and transinduction of the human β-globin locus

“…Chromosome pairing in Drosophila has been shown to influence gene expression in both positive and negative ways (for review, see Henikoff 1997). For example, the brown gene is trans-inactivated by insertion of heterochromatin into the brown gene on the other chromosome (Dreesen et al 1991). The two brown alleles are paired and the heterochromatin insertion in one causes both alleles to associate with the heterochromatic centromere, suppressing brown expression (Csink and Henikoff 1996;Dernburg et al 1996).…”

Enhancer Blocking and Transvection at the DrosophilaapterousLocus

“…To date, a number of trans-regulatory interactions that depend on chromosome pairing have been reported. For example, several types of pairing-dependent silencing have been observed, such as trans repression by the bw D mutant and pairing-sensitive silencing mediated by polycomb response elements (PREs) (Henikoff and Dreesen 1989;Dreesen et al 1991;Kassis 1994;Hagstrom et al 1997;Mü ller et al 1999;Sass and Henikoff 1999;Csink et al 2002). Another pairing-dependent regulatory interaction is the phenomenon of transvection, in which regulatory elements such as enhancers or silencers on one homolog can affect the expression of a gene in trans (Duncan 2002).…”

Copy number and orientation determine the susceptibility of a gene to silencing by nearby heterochromatin in Drosophila

“…Reduced sensitivity would result if insulator elements are associated with P[ bw+] but not with mini-white. The presence of insulator elements tightly associated with the brown gene is implied by the insensitivity of all P[bw+] transposons to position effects caused by insertions at various euchromatic sites (DREESEN et al 1991;MARTIN-MORRIS et al 1993). In contrast, mini-white lacks insulator elements, as evident from its extreme sensitivity to euchromatic position effects (KELLUM and SCHEDL 1991;CHUNC et al 1993;ROSEMAN et al 1993).…”

“…P[A2,3](99B) is a third chromosome insertion that encodes P transposase (ROBERTSON et al 1988). The line P[ bw+]92C is one of 32 previously reported nonvariegating lines (DREESEN et al 1991) produced by injection and subsequent mobilization of P-element constructs containing a wild-type genomic copy of the brown gene in the pDm24 vector (MISMER and RUBIN 1987); a moderately variegating derivative, V21, was produced by X-irradiation (DREESEN et al 1991). V21 is a translocation that places 2R heterochromatin -55-70 kb from the P[bw+] transposon ( Figure 1A) (J. SABL, unpublished results).…”

“…This dominant effect might be mediated by a heterochromatinsensitive transcription factor that makes direct contact with heterochromatic proteins (DREESEN et al 1991;~~ARTIN-MORRIS et al 1993;MARTIN-MORRIS and HENI-KOFF 1995). This hypothetical transcription factor would then interact with heterochromatin in a particular topological orientation.…”