DNA RECOGNITION BY BETA-SHEETS IN THE ARC REPRESSOR-OPERATOR CRYSTAL-STRUCTURE

TRANSCRIPTION of the ant gene during lytic growth of bacteriophage P22 (ref. 1) is regulated by the cooperative binding of two Arc repressor dimers to a 21-base-pair operator site2,3. Here we report the co-crys tal structure of this Arc tetramer-operator complex at 2.6 angstrom re solution. As expected from genetic4-6 and structural studies7 and from the co-crystal structure of the homologous Escherichia coli MetJ repr essor8, each Arc dimer uses an antiparallel beta-sheet to recognize ba ses in the major groove. However, the Arc and MetJ complexes differ in several important ways: the beta-sheet-DNA interactions of Arc are fa r less symmetrical; DNA binding by Arc is accompanied by important con formational changes in the beta-sheet; and Arc uses a different part o f its protein surface for dimer-dimer interactions.