ISWI, a member of the SWI2/SNF2 ATPase family, encodes the 140 kDa subunit of the nucleosome remodeling factor.

The generation of an accessible heat shock promoter in chromatin in vitro 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 SWI2/SNF2 multiprotein complex, a transcriptional activator that also appears to alter nucleosome structure. We have obtained protein microsequence and immunological evidence identifying the 140 kDa subunit of NURF as ISWI, previously of unknown function but highly related to SWI2/SNF2 only in 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 SWI2/SNF2 underscores these ATPases and their close relatives as key components of independent systems for chromatin remodeling.