ABSTRACT

In revolving biomotors, various models for dsDNA translocation have been defined by how many monomers are believed to have contact with the dsDNA substrate at any one time. The most widely accepted model for dsDNA substrate translocation in the phi29 dsDNA packaging motor is the “push-through-one-way-valve” model, in which dsDNA is pushed section-by-section through the central channel of the motor by means of ATPase and can enter but not exit (Fang et al., 2012; Zhang et al., 2012; Schwartz et al., 2013b; Zhao et al., 2013). In the “rotary inchworm” model, which is consistent with the “push-through-one-way-valve” model, activity is proposed to hand off from one monomer to the next in a single ring, with the inactivity of anyone subunit curtailing the entire process. In Rho and E1 helicases, the “spiral staircase” model consists of multiple subunits contacting the DNA/RNA substrate at any one time. The “limited escort” model for FtsK proposes three adjacent monomers having simultaneous contact with the dsDNA substrate. Research on SpoIIIE supports the “escort model,” with at least two adjacent subunits contacting the dsDNA substrate. Both the inchworm and partial escort models are highly consistent with the revolving motor mechanism. We will also explore the models to explain the directionality of dsDNA translocation in SpoIIIE. The traffic in revolving biomotors advances unidirectionally. In all dsDNA viruses, it was found that ATPase switches from low to high affinity for approaching dsDNA, so the strand attaches and detaches, effectively pushed forward to the next subunit, with a 30° left-handed twist of the channel wall. Channel loops act as a ratchet to prevent the DNA from reversing in its path toward the capsid in line with “push through one way valve” model.