Sequence elements in cis influence heterochromatic silencing in trans

The brown(Dominant) (bw(D)) allele contains a large insertion of heterochromatin, which causes the locus to aberrantly associate with heterochromatin in interphase nuclei and silences the wild-type allele in heterozygotes. Transgenes placed near the bw(+) locus, in trans to bw(D), can also be silenced. The strength of silencing (called trans inactivation) varies with the regulatory sequences of the transgene and its distance away from the bw(D) insertion site in trans. In this study, we examine endogenous sequences in cis that influence susceptibility of a reporter gene to trans inactivation. Flanking deletions were induced in two parental lines containing P-element transgenes showing trans inactivation of the mini-white reporter. These new lines, which have mini-white under the influence of different endogenous sequence elements, now show varied ability to be silenced by bw(D). Determination of the deleted regions and the levels of mini-white expression and trans inactivation has allowed us to explore the correlation between cis sequence elements and susceptibility to trans inactivation and to identify a 301-bp sequence that acts as an enhancer of trans inactivation. Intriguingly, this region encompasses the upstream regions of two divergently transcribed genes and contains a sequence motif that may bind BEAF, a protein involved in delimiting chromatin boundaries.