The Arabidopsis GCN5, ADA2a and ADA2b proteins are homologs of components of several yeast and animal transcriptional coactivator complexes. Previous work has implicated these plant coactivator proteins in the stimulation of cold-regulated gene expression by the transcriptional activator protein CBF1. Surprisingly, protein interaction studies demonstrate that the DNA-binding domain of CBF1 (and of a related protein, TINY), rather than its transcriptional activation domain, can bind directly to the Arabidopsis ADA2 proteins. The ADA2a and ADA2b proteins can also bind directly to GCN5 through their N-terminal regions (comparable to a region previously defined in yeast Ada2) and through previously unmapped regions in the middle of the ADA2 proteins, which bind to the HAT domain of GCN5. The ADA2 proteins enhance the ability of GCN5 to acetylate histones in vitro and enable GCN5 to acetylate nucleosomal histones. Moreover, GCN5 can acetylate the ADA2 proteins at a motif unique to the plant homologs and absent from fungal and animal homologs. We speculate that this modification may represent a novel autoregulatory mechanism for the plant SAGA-like transcriptional coactivator complexes.