Cocrystals of the DNA-binding domain of phage 434 repressor and a synthetic phage 434 operator.

The amino-terminal domain of the phage 434 repressor forms cocrystals with a synthetic phage 434 operator. The cocrystals diffract to at least 4 A, and x-ray crystallographic analysis of them is in progress. An analysis of the packing in the cocrystals shows that complexes consisting of dimers of amino-terminal domain bound specifically to operators are stacked end to end in longer protein-DNA rods parallel to the unit cell body diagonals. The DNA in the complexes has 10.5 base pairs per turn and a rise per base of 3.26 A--values consistent with B-form DNA--indicating that DNA is neither unwound nor overwound by bound repressor. The packing analysis suggests an approach that might facilitate the cocrystallization of other DNA-binding proteins with the DNA they recognize.