Members of the R/B basic helix-loop-helix (bHLH) family of plant transcription factors are involved in a variety of growth and differentiation processes. We isolated a dominant mutation in an R/B-related bHLH transcription factor in the course of studying Arabidopsis tryptophan pathway regulation. This mutant, atr2D, displayed increased expression of several tryptophan genes as well as a subset of other stress-responsive genes. The atr2D mutation creates an aspartate to asparagine change at a position that is highly conserved in R/B factors. Substitutions of other residues with uncharged side chains at this position also conferred dominant phenotypes. Moreover, overexpression of mutant atr2D, but not wild-type ATR2, conferred pleiotropic effects, including reduced size, dark pigmentation, and sterility. Therefore, atr2D is likely to be an altered-function allele that identifies a key regulatory site in the R/B factor coding sequence. Double-mutant analysis with atr1D, an overexpression allele of the ATR1 Myb factor previously isolated in tryptophan regulation screens, showed that atr2D and atr1D have additive effects on tryptophan regulation and are likely to act through distinct mechanisms to activate tryptophan genes. The dominant atr mutations thus provide tools for altering tryptophan metabolism in plants.