Catalytic component of the RPD3 histone deacetylase complexes RPD3C(L) and RPD3C(S) responsible for the deacetylation of lysine residues on the N-terminal part of the core histones (H2A, H2B, H3 and H4). Histone deacetylation gives a tag for epigenetic repression and plays an important role in transcriptional regulation, cell cycle progression and developmental events. SIN3 has also a RPD3 independent function required for normal longevity. Component of the RPD3C(L) complex composed of at least ASH1, CTI6, DEP1, PHO23, RPD3, RXT2, RXT3, SAP30, SDS3, SIN3, UME1 and UME6. Component of the RPD3C(S) complex composed of at least EAF3, RCO1, RPD3, SIN3, and UME1. Interacts with ESS1 and STB1. Present with 1660 molecules/cell in log phase SD medium.