UC Berkeley

Cellular replication forks are universally powered by ring-shaped, hexameric helicases that encircle and unwind DNA. To better understand the molecular mechanisms and control of these enzymes, we used an ensemble of methods to investigate the bacterial replicative helicase, DnaB. A 3.3Å crystal structure of Aquifex aeolicus DnaB complexed with nucleotide reveals a new conformational state for this family of motor proteins. Electron microscopy and small angle X-ray scattering studies confirm the state seen crystallographically, showing that the DnaB ATPase domains and an associated N-terminal collar transition between two physical states in a nucleotide-dependent manner. Mutant helicases locked into either collar state are active, but display different capacities to support critical activities such as duplex translocation and primase-dependent RNA synthesis. Our findings establish the DnaB collar as a key auto-regulatory hub that controls the ability of the helicase to transition between different functional states in response to nucleotide and both replication initiation and elongation factors.