Mechanisms of a ring shaped helicase

Bacteriophage T7 helicase (T7 gene 4 helicase-primase) is a prototypical member of the ring-shaped family of helicases, whose structure and biochemical mechanisms have been studied in detail. T7 helicase assembles into a homohexameric ring that binds single-stranded DNA in its central channel. Using RecA-type nucleotide binding and sensing motifs, T7 helicase binds and hydrolyzes several NTPs, among which dTTP supports optimal protein assembly, DNA binding and unwinding activities. During translocation along single stranded DNA, the subunits of the ring go through dTTP hydrolysis cycles one at a time, and this probably occurs also during DNA unwinding. Interestingly, the unwinding speed of T7 helicase is an order of magnitude slower than its translocation rate along single stranded DNA. The slow unwinding rate is greatly stimulated when DNA synthesis by T7 DNA polymerase is coupled to DNA unwinding. Using the T7 helicase as an example, we highlight critical findings and discuss possible mechanisms of helicase action.