Mechanisms of transcriptional regulation by a Drosophila insulator protein. Alexey A. Soshnev, Pamela K. Geyer. Molec & Cellular Biol, Univ Iowa, Iowa City, IA.

   Insulators are DNA elements that bind protein complexes and constrain the action of enhancers and silencers. The Drosophila genome encodes several insulator proteins, including the model insulator protein, Suppressor of Hairy-wing [Su(Hw)]. This zinc finger (ZnF) DNA binding protein binds the gypsy retrotransposon, as well as over 3000 constitutively occupied endogenous non-gypsy sites. At the gypsy insulator, Su(Hw) recruits Centrosomal Protein of 190 kD (CP190) and Modifier of mdg4 67.2 kD isoform (Mod67.2), two partner proteins required for gypsy insulator function. Surprisingly, CP190 and Mod67.2 localize to fewer than a third of endogenous Su(Hw) binding sites (SBSs), and their loss of function phenotypes are distinct from loss of Su(Hw). These observations imply that the regulatory roles of Su(Hw) extend beyond formation of endogenous insulators. To gain a better understanding of the Su(Hw) function, we defined transcriptional requirements for this protein in the ovary, as this is the only tissue where Su(Hw) function is essential. Our studies revealed that Su(Hw) is a direct transcriptional regulator, with the majority of Su(Hw) target genes upregulated upon Su(Hw) loss. Most of these upregulated genes display enriched expression in the central nervous system, suggesting that Su(Hw) is a repressor of neuronal genes in non-neuronal tissues. Several findings are consistent with this prediction. First, Su(Hw) does not accumulate in post-mitotic neurons. Second, su(Hw) mutants are temperature sensitive, a phenotype consistent with upregulation of neuronal Su(Hw) target genes. Third, ectopic expression of Su(Hw) is associated with developmental defects and cell death. Based on these data, we propose that Su(Hw) may represent a functional homologue of the vertebrate RE1-Silencing Transcription Factor (REST), a ZnF transcription factor that acts as a repressor of neuronal genes in non-neuronal tissues. Investigations into the repressor function of Su(Hw) will provide insights into how this protein achieves multiple transcriptional regulatory roles.