VISUALIZATION OF CHROMOSOMAL DOMAINS WITH BOUNDARY ELEMENT-ASSOCIATEDFACTOR BEAF-32

We have purified two proteins from Drosophila that bind to the scs' bo undary element of the hsp70 domain at locus 87A7. Their palindromic bi nding sites (CGATA-TATCG) symmetrically abut the previously mapped hyp ersensitive site of scs'. We have cloned a cDNA for one of these prote ins, BEAF-32 (boundary element-associated factor of 32 kDa). It encode s a novel protein that is bound to scs' but not scs in vivo. Immunosta ining localizes BEAF to hundreds of interbands and many puff boundarie s on polytene chromosomes, suggesting that a chromosomal domain consis ts of a band (or puff) and part of the flanking interbands. Enhancer b locking assays implicate the palindromic binding site in boundary func tion. The lack of enhancer blocking in transiently transfected cells s uggests an involvement of chromatin, nuclear structure, or both in bou ndary function.