The petunia NAM and ArabidopsisATAF1 and CUC2 genes define the conserved NAC domain. The NAC domain is widely distributed in plants but so far, has not been found in other eukaryotes. Genetic analysis of mutant alleles and expression assays proved that NAC domain proteins comprise at least one component of the SAM specification network. The molecular function of the NAC domain containing proteins was first suggested by two Arabidopsis genes, ATAF1 and ATAF2. Hirt (personal communication) reported that both can trans-activate a CaMV35S promoter when expressed in Saccharomyces cerevisiae,suggesting that NAC genes act at the transcriptional level. Recent studies further support the conjecture that NAC genes function at the transcriptional level (Xie et al., 1999, 2000; Aida et al., 1999). Here we report the isolation of a NAC domain gene, AtNAM, from an Arabidopsis developing seed cDNA library. In vitro and in vivo assays in yeast provided experimental evidence that AtNAM encodes a transcription factor and that the NAC domain is a putative DNA-binding domain (DBD). The DBD is contained within a 60 amino acid region which potentially folds into a helix-turn-helix motif that specifically binds to a CaMV 35S promoter region. The portion of the protein that is transcriptionally active is located in the C-terminal region of the protein, a highly divergent region among NAC genes. The arabidopsis genome contains more than 90 predicted NAC domain genes; we refer to these collectively as the AtNAC superfamily. The first two exons of all members of this superfamily encode the NAC domain. Most AtNAC genes contain three exons with the last exon encoding an activation domain. A subfamily of AtNAC genes contain additional terminal exons that may encode protein domains that expand the range of biological functions of the proteins encoded by the NAC domain superfamily.