Transcriptional repression of ci in the posterior compartment could be initiated by en and maintained by ph , the ph expression depending itself on en expression. These cells might now produce enough Ci and Ptc to become competent to receive the Hh signal. If posterior cells are not competent to receive an Hh signal, higher amounts of Hh would not affect the posterior cells. Each polyhomeotic gene consists of five exons. The polyhomeotic locus of Drosophila is a complex locus essential for the maintenance of segmental identity.
Genetic analysis suggests that two independent units contribute to ph function.
Comparison of genomic sequence shows that the ph locus has been duplicated, and that it contains proximal and distal transcription units. The distal protein is very similar to the proximal product, except for the absence of an amino terminal region, and a small region near the carboxy terminus. Comparison to the mammalian ph homolog rae28 reveals two more regions of similarity, indicated as H1 and H2. Drosophila Sex comb on midleg protein, another PcG protein, contains a domain homologous to Ph corresponding to the H2 region. Single copies of an approximately 65-70 residue domain are present in the sequences of 14 eukaryotic proteins, including yeast byr2, STE11, ste4, and STE50, which are essential participants in sexual differentiation.