(A) The COMPASS complex consists of six subunits including the enzyme Set1 (green) which methylates histone H3. When COMPASS binds to the nucleosome (not shown), three of its subunits (Swd3, Spp1 and Sd1) are slightly rotated towards the nucleosome (in the direction of the arrow; original positions shown in grey). (B) As well as rotating these three subunits, ubiquitination of the histone H2B (H2Bub; red) allows COMPASS to properly bind to the nucleosome and contact the DNA (black) at three distinct locations, including two which are at opposing ends of the nucleosome (indicated by *). Together these conformational changes help place the N-terminal tail of histone H3 (red dashed line) into the active site of Set1 so that H3 can be methylated. (C) Comparing COMPASS complexes which are bound to ubiquitinated (colored diagram) and non-ubiquitinated nucleosomes (light grey diagram) revealed that ubiquitin allows COMPASS to be in a better position for methylating the N-terminal tail of histone H3.