We previously reported five DNA bend sites (ERB-4 to -1, and ERB+1) in the promoter region of the human estrogen receptor alpha (ERalpha) gene [FEBS Lett. 444 (1999) 117]. One of these sites, ERB-2, was accompanied by two half motifs of the estrogen response element (ERE) and several short poly(dA)(.)poly(dT) tracts including an A(4) tract located next to a half ERE motif. This A(4) tract and the 20 bp immediate flanking sequence containing a half ERE motif (T3B) exhibited DNA curvature. Transcription assays using luciferase as a reporter gene indicated that T3B sequence conferred positive estrogen responsiveness. Mutations introduced in this sequence indicated that both bendability and estrogen responsiveness were synergistically associated with the A(4) tract located next to the half ERE motif. This motif and a mutant sequence, GGTTA, had affinity for ERalpha protein, which seems to account for ERalpha protein binding to the region without an ERE motif. These findings suggest that some DNA curvature acts as a transcriptional modulator by modifying the state of ligand effects.