The yeast BAS1 protein is a transcriptional activator with an amino-terminal domain homologous to the DNA-binding domain of the oncoprotein Myb containing three imperfect tryptophan-rich repeats. In contrast to Myb-related transcription factors from higher eukaryotes, where the second and third repeat constitutes a minimal independent DNA-binding domain, all three repeats of BAS1 were found to be necessary for sequence-specific DNA binding. Moreover, an active DNA-binding subdomain was obtained only if the first repeat was enlarged in the amino-terminal direction to include 3 tryptophans and a 23-amino acid insertion and if 55 amino acids carboxyl-terminal to the third repeat were included. The BAS1 DNA-binding site was analyzed in detail and found to cover 8-9 base pairs with no similarity to the Myb recognition element. The binding site included a conserved hexameric TGACTC motif, the methylation of which abolished BAS1 binding, as well as a 3-base pair extension that seemed to have a modulatory effect on BAS1 affinity and where binding was less affected by methylation.