Abstact

The structure of a new crystal form (space group C2), grown at pH 8.0 and diffracting to 1.95 A resolution, of the replicative homo-hexameric DNA helicase RepA encoded by plasmid RSF1010 is reported. In contrast to previous crystals grown at pH 6.0 in space group P2(1) (Niedenzu et al., 2001), only one half (a trimer) of the RepA hexamer occupies the asymmetric unit of the space-group C2 crystals. The new crystal packing explains the pH-dependent hexamer-hexamer association mechanism of RepA. The C-terminus (264)VLERQRKSKGVPRGEA(279), which could not be modelled in the previous structure, is clearly defined in the present electron density except for the last four amino acids. Sulfate anions occupy the six ATPase active sites of RepA at positions where the product phosphates are supposed to bind. Binding of sulfate anions induces conformational changes both at the ATPase active sites and throughout the whole molecular structure. In agreement with electron microscopy, the above studies implicate structural changes to an "open" form that may occur upon binding and hydrolysis of nucleotide 5'-triphosphates and could be essential for DNA duplex-unwinding activity.