Histone chaperone-dependent acetylase that modifies 'Lys-56' of histone H3 (H3K56ac) (PubMed:20656950). Histone H3 'Lys-56' acetylation may be required for S-phase-linked DNA damage tolerance (By similarity). Also acetylates 'Lys-9' of histone H3 (H3K9ac) (By similarity). Autoacetylates (PubMed:20656950). acetyl-CoA + L-lysyl-[protein] = CoA + H(+) + N(6)-acetyl-L-lysyl-[protein] acetyl-CoA + L-lysyl-[histone] = CoA + H(+) + N(6)-acetyl-L-lysyl-[histone] Forms a complex composed of two RTT109 subunits and one VPS75 homodimer (By similarity). Interacts with ASF1 (By similarity). Belongs to the RTT109 family.