YEAST GCN5 FUNCTIONS IN 2 MULTISUBUNIT COMPLEXES TO ACETYLATE NUCLEOSOMAL HISTONES - CHARACTERIZATION OF AN ADA-COMPLEX AND THE SAGA-(SPT ADA)-COMPLEX/

The transcriptional adaptor protein Gcn5 has been identified as a nucl ear histone acetyltransferase (HAT). Although recombinant yeast Gcn5 e fficiently acetylates free histones, it fails to acetylate histones co ntained in nucleosomes, indicating that additional components are requ ired for acetylation of chromosomal histones. We report here that Gcn5 functions as a catalytic subunit in two high-molecular-mass native HA T complexes, with apparent molecular masses of 0.8 and 1.8 megadalton (MD), respectively, which acetylate nucleosomal histones. Both the 0.8 - and 1.8-MD Gcn5-containing complexes cofractionate with Adn2 and are lost in gcn5 Delta, ada2 Delta, or ada3 Delta yeast strains, illustra ting that these HAT complexes are bona fide native Ada-transcriptional adaptor complexes. Importantly, the 1.8-MD adaptor/HAT complex also c ontains Spt gene products that are linked to TATA-binding protein (TBP ) function. This complex is lost in spt20/ada5 Delta and spt7 Delta st rains and Spt3, Spt7, Spt20/Ada5, Ada2, and Gcn5 all copurify with thi s nucleosomal HAT complex. Therefore, the 1.8-MD adaptor/HAT complex i llustrates an interaction between Ada and Spt gene products and confir ms the existence of a complex containing the TBP group of Spt proteins as demonstrated by genetic and biochemical studies. We have named thi s novel transcription regulatory complex SAGA (Spt-Ada-Gcn5-Acetyltran sferase). The function of Gcn5 as a histone acetyltransferase within t he Ada and SAGA adaptor complexes indicates the importance of histone acetylation during steps in transcription activation mediated by inter actions with transcription activators and general transcription factor s (i.e., TBP).