RecBCD, an enzyme that unwinds the DNA double helix so that it can be copied or repaired, is powered by two motor units that run in opposite directions, according to a report by UC Davis researchers in the June 19 issue of Nature.

RecBCD is one of a class of motor proteins that travel along either DNA or other protein tracks, said Stephen Kowalczykowski, professor of microbiology at UC Davis. In 2001, Kowalczykowski and colleagues used advanced microscopy techniques to film a single molecule of RecBCD unwinding DNA in real time.

The ends of a single strand of DNA are called three-prime (3') and five-prime (5') respectively, because of the way the sugar molecules that form the backbone of the DNA are joined together. When two strands of DNA are twisted into a double helix, they run in opposite directions, so that the 3' end of one strand is next to the 5' end of the other.

RecBCD consists of three different proteins, of which RecB was known to act as a motor that traveled and unwound DNA from the 3' to 5'. Kowalczykowski and postdoctoral researchers Mark Dillingham and Maria Spies have now shown that the RecD subunit functions as a motor that travels in the opposite direction (5' to 3'). The motors, one on each strand of double-stranded DNA, therefore push the RecBCD complex in the same overall direction.

"Having two motors allows it to go a long distance before dropping off, as one can hand off to the other if it is temporarily derailed," for example by a gap or a roadblock in one strand, Kowalczykowski said.