Actin-related proteins (Arps), which share a basal structure with actin but have distinct functions, have been found in a wide variety of organisms. While their functions are not yet clear, some Arps are localized in the nucleus and are suggested to contribute to the regulation of transcription. An essential gene of Saccharomyces cerevisiae, Act3p/Arp4, encodes the first identified nuclear Arp, which has been shown to bind to core histones in vitro. Here we have analyzed the in vivo function of Act3p/Arp4 on the his4-912delta promoter. Chromatin immunoprecipitation assays show that Act3p/Arp4 is bound to the entire his4-912delta promoter region. Conditional act3/arp4 mutations affect transcription from the his4-912delta promoter, where decreased Act3p/Arp4 binding and a change in nuclease sensitivity of chromatin were observed, showing the involvement of Act3p/Arp4 in the regulation of gene expression through the organization of chromatin structure. Taken together with the presence of Act3p/Arp4 in chromatin remodeling and histone acetyltransferase complexes, it is suggested that Act3p/Arp4 functions in transcriptional regulation to recruit chromatin remodeling and histone acetyltransferase complexes onto chromatin.