Regulation of the genes of the bithorax complex in Drosophila.

The BX-C is a set of master control genes that trans-regulate other genes and thereby control much of the segmentation pattern of the fly. The BX-C genes are themselves regulated in cis and trans. Three rules governing cis-regulation of BX-C are applicable over a region extending from Ubx to at least iab-7, a distance of nearly 300 kb on the DNA map: The colinearity (COL) rule: genes are colinear with respect to map location and order of expression along the body axis, the only exception thus far being pbx+; the cis-inactivation (CIN) rule: a mutant lesion in one gene tends to cis-inactivate the wild-type gene(s) immediately distally; and the cis-overexpression (COE) rule: certain mutant lesions in a given gene cause the next most proximal gene to overexpress one segment more anterior to the one in which the latter gene normally expresses. A model is proposed that attempts to account for these rules by invoking a special cis-regulatory entity (E) that diffuses more efficiently along the chromosome than between chromosomes.