Crystal structure of T7 gene 4 ring helicase indicates a mechanism for sequential hydrolysis of nucleotides

We have determined the crystal structure of arl active, hexameric fragment of the gene 4 helicase from bacteriophage T7. The structure reveals how sub unit contacts stabilize the hexamer. Deviation from expected six-fold symme try of the hexamer indicates that the structure is of an intermediate on th e catalytic pathway. The structural consequences of the asymmetry suggest a "binding change" mechanism to explain how cooperative binding and hydrolys is of nucleotides are coupled to conformational changes in the ring that mo st likely accompany duplex unwinding. The structure of a complex with a non hydrolyzable ATP analog provides additional evidence for this hypothesis, w ith only four of the six possible nucleotide binding sites being occupied i n this conformation of the hexamer. This model suggests a mechanism for DNA translocation.