The ISWI chromatin-remodeling protein is required for gene expression and the maintenance of higher order chromatin structure in vivo.

Drosophila ISWI, a highly conserved member of the SWI2/SNF2 family of ATPases, is the catalytic subunit of three chromatin-remodeling complexes: NURF, CHRAC, and ACF. To clarify the biological functions of ISWI, we generated and characterized null and dominant-negative ISWI mutations. We found that ISWI mutations affect both cell viability and gene expression during Drosophila development. ISWI mutations also cause striking alterations in the structure of the male X chromosome. The ISWI protein does not colocalize with RNA Pol II on salivary gland polytene chromosomes, suggesting a possible role for ISWI in transcriptional repression. These findings reveal novel functions for the ISWI ATPase and underscore its importance in chromatin remodeling in vivo.