ATP binding flips RECQ1 from strand-annealing oligomer to strand-unwinding monomers or dimers.

VINDIGNI/PLOS

The RECQ1 helicase can unwind DNA or stitch it back together. Which job the protein performs depends on how much help it gets, say Laura Muzzolini, Alessandro Vindigni (International Centre for Genetic Engineering and Biotechnology, Trieste, Italy), and colleagues. Unwinding is the job of single molecules or dimers, whereas oligomers of five or six enzymes take on annealing.

RECQ1 peels open DNA to permit replication and transcription. Why it also reseals strands isn't certain, but it may be assisting with repair. A further mystery is how the same molecule pulls off these two opposing tasks.

The team found that RECQ1 comes in two versions: a small form that's a monomer or a dimer; and a hefty size composed of pentamers or hexamers. Mixing in single-stranded DNA pushed the balance toward the larger version, but ATP favored the smaller one. Scanning electron microscopy of the larger oligomer showed a three-ringed structure that falls apart upon addition of ATP. Together, these findings suggest that the larger oligomer is responsible for annealing and the small version for unwinding.

The researchers bolstered that inference with competition experiments. If enzymes band together to open up DNA, adding nonfunctional mutant RECQ1 under single-turnover conditions should slow unwinding. The mutant had no effect, indicating that RECQ1 works to unwind DNA either alone or in tightly bound pairs.

Reference:

Muzzolini, L., et al.
2007
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PLoS Biol.
doi:.