Drosophila Genes in Development: Polycomb-group
Polycomb family proteins work together as a constituent of chromatin to establish gene repression (Cheng N. N., 1994).
polyhomeotic is a complex locus encoding two transcription factors: Polyhomeotic-proximal (PH-P) and Polyhomeotic-distal (PH-D). These proteins are a part of the Polycomb group (Pc-G) that represses homeotic gene expression, and thus help maintain segment identity in the developing fly (McKeon J., 1991).
polyhomeotic is involved in a negative autoregulatory loop. As the number of polyhomeotic copies in the genome increases, the level of transcription of each decreases (Fauvarque M. O., 1995).
The PH-P protein has 4 glutamine rich regions, a C4 zinc finger domain, a C-terminal alpha helix motif and a serine and threonine rich domain (de Camillis M.A., 1992).
Nuclear
Early expression in the cellular blastoderm appears in two domains, an anterior domain extending from 40 to 50% egg length (measured from the posterior) and a posterior domain extending from 30 to 10% egg length. The region anterior to the cephalic furrow is more intensely stained than the posterior (Fauvarque M. O., 1995).
In germ-band extended embryos, polyhomeotic is expressed in most if not all cells of the presumptive neuroectoderm and epidermis. During gastrulation, ph expression evolves rapidly to form a striped pattern, similar to that of Engrailed in germ-band extended embryos (Serrano N., 1995). Of particular interest is the activation of polyhomeotic by Engrailed. When examined at stage 10, the expression domains of en and ph are found to overlap. At this stage ph is expressed in 14 evenly spaced bands in the anterior compartment of each parasegment, coincident with engrailed expression domains. Engrailed binding sites exist in two regions between ph-p and ph-d and in the one promoter site proximal to ph-d.
At some time after the initial ph activation, ph expression becomes independent of EN. There is a transition period when maintenance of en expression is actually dependent on ph. During the larval stage, EN activates ph expression during wing morphogenesis, even as it represses ph expression in the hindgut. It would appear that EN can act as either an activator or as a repressor of ph expression. It appears that engrailed, a gene whose activity is essential in the correct formation of segment polarity, has coopted PH as its own private Polycomb-group protein, adding a segment polarity element to gene silencing.
In larvae, Engrailed activates polyhomeotic expression during wing morphogenesis (Serrano N., 1995). Larval expression occurs in the eye-antennal disc, the metathoracic leg disc, the brain and anterior midgut (Fauvarque M. O., 1995).
Pc protein and polyhomeotic protein have exactly the same binding patterns on polytene chromosomes. (Franke A., 1992) ph-p protein binds to about 80 sites along the polytene chromosomes that are distributed throughout the genome. The bound sites largely overlap those bound by Pc protein. ph-p binds to insertion sites of constructs containing sequences from the bxd region of Ubx showing that the binding is DNA-sequence dependent. (de Camillis M.A., 1992)
Pair-rule
HOX-Pro