ADA3, A PUTATIVE TRANSCRIPTIONAL ADAPTER, CONSISTS OF 2 SEPARABLE DOMAINS AND INTERACTS WITH ADA2 AND GCN5 IN A TRIMERIC COMPLEX

Mutations in yeast ADA2, ADA3, and GCN5 weaken the activation potentia l of a subset of acidic activation domains. In this report, we show th at their gene products form a heterotrimeric complex in vitro, with AD A2 as the linchpin holding ADA3 and GCN5 together. Further, activation by LexA-ADA3 fusions in vivo are regulated by the levels of ADA2. Com bined with a prior observation that LexA-ADA2 fusions are regulated by the levels of ADA3 (N. Silverman, J. Agapite, and L. Guarente, Proc. Natl. Acad. Sci. USA 91:11665-11668, 1994), this finding suggests that these proteins also form a complex in cells. ADA3 can be separated in to two nonoverlapping domains, an amino-terminal domain and a carboxyl -terminal domain, which do not separately complement the slow-growth p henotype or transcriptional defect of a Delta ada3 strain but together supply full complementation. The carboxyl-terminal domain of ADA3 alo ne suffices for heterotrimeric complex formation in vitro and activati on of LexA-ADA2 in vivo. We present a model depicting the ADA complex as a coactivator in which the ADA3 amino terminal domain mediates an i nteraction between activation domains and the ADA complex.