FIG. 6.

Transcriptional remodeling of CMMs. Initially, gap and segmentation genes set up the expression pattern of the homeotic genes. CMMs are subsequently required to maintain the preset homeotic gene expression patterns throughout development by distinguishing transcriptionally active and silenced chromatin. Transcription through a CMM switches the element from the silenced to activated state. We can envisage two possible, not mutually exclusive, functions to achieve this heritable switch. In model A, the RNA polymerase (Pol) II complex passes through CMMs in intergenic regions, displacing the repressive complexes. As a consequence, chromatin becomes accessible for further epigenetic modifications. In model B, RNA polymerase II “piggybacks” remodeling complexes which epigenetically mark histones. These modifications are stably sustained throughout development and inherit an open chromatin conformation (ovals, nucleosomes; half-circles, silencing complexes; Mod, epigenetic modification of histones like histone acetyltransferase [HAT], histone deacetylase [HDAC], or methyltransferase [MT]).