Pulling apart DNA

After taking a closer look under the hood of an enzyme motor that unwinds double-stranded DNA, researchers have reported a new "spring-loaded" mechanism for its unraveling action (Science, DOI: 10.1126/science.1144130). Helicase enzymes normally pull apart DNA to allow genome-duplicating and protein-making machinery to access the genome. On the basis of fluorescence resonance energy transfer experiments, Sua Myong and Taekjip Ha of the University of Illinois, Urbana-Champaign, and collaborators propose that the motor moves along the DNA one base pair at a time, each step catalyzed by one ATP molecule. During the course of three sequential base-pair steps, mechanical tension builds up within the enzyme that is then used to separate the DNA that has just been traversed. The concurrent unwinding of three base pairs conveniently corresponds to the length of an amino acid codon, Ha notes. The researchers made the discovery while studying the helicase motor of hepatitis C, but Ha says the mechanism may be more universal.