Differential expression of the Drosophila BX-C in polytene chromosomes in cells of larval fat bodies: a cytological approach to identifying in vivo targets of the homeotic Ubx, Abd-A and Abd-B proteins.

We have analyzed the expression of homeotic Bithorax Complex proteins in the fat bodies of Drosophila larvae by staining with specific antibodies. We have found that these proteins are differentially expressed along the anteroposterior (AP) axis of the fat body, with patterns parallel to those previously characterized for the larval and adult epidermis. As fat body nuclei have polytene chromosomes, we were able to identify the BX-C locus and show that it assumes a strongly puffed conformation in cells actively expressing the genes of the BX-C. Immunostaining of these polytene chromosomes provided the resolution to cytologically map binding sites of the three proteins: Ubx, Abd-A and Abd-B. The results of this work provide a system with which to study the positioning of chromatin regulatory proteins in either a repressed and/or active BXC at the cytological level. In addition, the results of this work provide a map of homeotic target loci and thus constitute the basis for a systematic identification of genes that are direct in vivo targets of the BX-C genes.