The polyhomeotic gene of Drosophila encodes a chromatin protein that shares polytene chromosome-binding sites with Polycomb.

Abstract

The Polycomb group (PcG) genes in Drosophila melanogaster are required for maintenance of correct spatial expression of homeotic genes, and their products are thought to form either a regulatory network or act as a multimeric complex. Recently, it has been suggested that because of homology between Polycomb (Pc) and Su(var)205, PcG genes encode chromatin proteins required for the maintenance of a determined state in chromatin. The polyhomeotic {ph) gene is a member of the PcG of genes. We present DNA sequence of a ph cDNA, which encodes a 169-kD protein with a single putative zinc finger, a serine/threonine-rich region, and has glutamine repeats, suggesting that ph is a DNA-binding protein. Polyclonal antisera directed against ph protein bind to -80 sites on polytene chromosomes. Most of these sites appear to be the same as those recognized by antibodies to Pc protein, ph protein binds to insertion sites of constructs containing DNA from the bithoraxoid {bxd) region of the Bithorax complex, showing that ph binding to chromatin is DNA dependent. The same bxd constructs are recognized by Pc protein, strongly supporting the hypothesis that ph and Pc interact directly.

polyhomeotic regulatory sequences induce developmental regulator-dependent variegation and targeted P-element insertions in Drosophila.

“…One of these sites corresponds to the ph locus itself (Zink and Paro 1989;DeCamillis et al 1992). Therefore, we tested the effect of ph and Pc mutations on ph-induced variegated eye phenotype.…”

“…Surprisingly >50% (11) are localized in the same polytene chromosomal section as a PH/PC protein-binding site (Zink and Paro 1989;DeCamillis et al 1992), which suggests that P~ph] inserts nonrandomly and preferentially into PH/PC target genes. Another proof of the directed nature of the insertion events is that two locations (12D and 35C) had two and three independent inserts, respectively.…”

“…We note that two of these four lines do not variegate, which suggests that preferential insertion can occur independently of the variegated phenotype. Therefore it seems clear that an unusually high proportion of genes acting in aPH/PC protein-binding sites are defined as in Zink and Paro (1989) and DeCamillis et al (1992). bNonvariegating lines mapped for their particular phenotype.…”

“…The PH/PC target sites include the two homeotic complexes but also several loci corresponding to trx-G and Pc-G genes (Zink and Paro 1989;Zink et al 1991;DeCamillis et al 1992). In particular, the locus 2D, where ph is located, is included in PH/PC target sites, indicating that ph expression may be regulated by PH and PC proteins.…”

“…In particular, the locus 2D, where ph is located, is included in PH/PC target sites, indicating that ph expression may be regulated by PH and PC proteins. PH and PC proteins were shown to bind chromatin in a DNA-specific manner, although neither PH nor PC was shown to bind directly to naked DNA (Zink et al 1991;DeCamillis et al 1992). As they coprecipitate with -10-15 other proteins, suggesting that they act in a multimeric complex, it is likely that at least one of the other components binds to the DNA .…”