ISWI, A MEMBER OF THE SW12 SNF2 ATPASE FAMILY, ENCODES THE 140-KDA SUBUNIT OF THE NUCLEOSOME REMODELING FACTOR/

The generation of an accessible heat shock promoter in chromatin in vi tro requires the concerted action of the GAGA transcription factor and NURF, an ATP-dependent nucleosome remodeling factor. NURF is composed of four subunits and is biochemically distinct from the SW12/SNF2 mul tiprotein complex, a transcriptional activator that also appears to al ter nucleosome structure. We have obtained protein microsequence and i mmunological evidence identifying the 140 kDa subunit of NURF as ISWI, previously of unknown function but highly related to SW12/SNF2 only i n the ATPase domain. The ISWI protein is localized to the cell nucleus and is expressed throughout Drosophila development at levels as high as 100,000 molecules/cell. The convergence of biochemical and genetic studies on ISWI and SW12/SNF2 underscores these ATPases and their clos e relatives as key components of independent systems for chromatin rem odeling.