The ACF chromatin remodeling complex is essential for Polycomb repression

Establishing and maintaining appropriate gene repression is critical for the health and development of multicellular organisms. Histone H3 lysine 27 (H3K27) methylation is a chromatin modification associated with repressed facultative heterochromatin, but the mechanism of repression remains unclear. We used a forward genetic approach to identify novel genes involved in transcriptional silencing of H3K27 methylated chromatin in the filamentous fungus, Neurospora crassa. We found crf4-1Iswi and crf4-2Itc1/Acf1 are required for repression of a subset of H3K27 methylated genes and that their protein products interact to form an ACF-like chromatin remodeling complex. This N. crassa ACF-like complex is promiscuous, yet can target facultative heterochromatin, where it is responsible for pulling the +1 nucleosome in the 5' direction. H3K27 methylated genes that are up-regulated when crf4-1Iswi or crf4-2Itc1/Acf1 are deleted show a downstream shift of the +1 nucleosome suggesting that proper nucleosome positioning is important for repression of facultative heterochromatin. Overall design: MNase-seq, H3K27 methylation ChIP-seq, RNA-seq and DamID with Neurospora crassa.