ADA1, a novel component of the ADA/GCN5 complex, has broader effects than GCN5, ADA2, or ADA3

The ADA genes encode factors which are proposed to function as transcriptional coactivators. Here we describe the cloning, sequencing, and initial characterization of a novel ADA gene, ADA1. Similar to the previously isolated ada mutants, ada1 mutants display decreases in transcription from various reporters. Furthermore, ADA1 interacts with the other ADAs in the ADA/GCN5 complex as demonstrated by partial purification of the complex and immunoprecipitation experiments. We estimate that the complex has a molecular mass of approximately 2 MDa. Previously, it had been demonstrated that ada5 mutants displayed more severe phenotypic defects than the other ada mutants (G. A. Marcus, J. Horiuchi, N. Silverman, and L. Guarente, Mol. Cell. Biol. 16:3197-3205, 1996; S. M. Roberts and F. Winston, Mol. Cell. Biol. 16:3206-3213, 1996). ada1 mutants display defects similar to those of ada5 mutants and different from those of the other mutants with respect to promoters affected, inositol auxotrophy, and Spt- phenotypes. Thus, the ADAs can be separated into two classes, suggesting that the ADA/GCN5 complex may have two separate functions. We present a speculative model on the possible roles of the ADA/GCN5 complex.