WHITE(+) TRANSGENE INSERTIONS PRESENTING A DORSAL VENTRAL PATTERN DEFINE A SINGLE CLUSTER OF HOMEOBOX GENES THAT IS SILENCED BY THE POLYCOMB-GROUP PROTEINS IN DROSOPHILA-MELANOGASTER/

We used the white gene as an enhancer trap and reporter of chromatin s tructure. We collected white(+) transgene insertions presenting a pecu liar pigmentation pattern in the eye: white expression is restricted t o the dorsal half of the eye, with a clear-cut dorsal/ventral (D/V) bo rder. This D/V pattern is stable and heritable, indicating that phenot ypic expression of the white reporter reflects positional information in the developing eye. Localization of these transgenes led us to iden tify a unique genomic region encompassing 140 kb in 69D1-3 subject to this D/V effect. This region contains at least three closely related h omeobox-containing genes that are constituents of the iroquois complex (IRO-C). IRO-C genes are coordinately regulated and implicated in sim ilar developmental processes. Expression of these genes in the eye is regulated by the products of the Polycomb-group (Pc-G) and trithorax-g roup (trx-G) genes but is not modified by classical modifiers of posit ion-effect variegation. Our results, together with the report of a Pc- G binding site in 69D, suggest that we have identified a novel cluster of target genes for the Pc-G and trx-G products. We thus propose that ventral silencing of the whole IRO-C in the eye occurs at the level o f chromatin structure in a manner similar to that of the homeotic gene complexes, perhaps by local compaction of the region into a heterochr omatin-like structure involving the Pc-G products.