Single-Molecule Compaction of DNA

Cindy Magee

Trying to segregate disorganized chromosomes is a feat akin to unpacking tangled Christmas lights. But through largely unknown processes, proteins called condensins help organize DNA during cell division. Using optical-trap microscopy, researchers recently analyzed an Escherichia coli condensin, MukBEF, which works like a "molecular Velcro," says Carlos Bustamante, a professor at the University of California, Berkeley.

Bustamante and colleagues mechanically manipulated a single molecule of DNA held on a bead, revealing that the addition of purified MukBEF resulted in DNA compaction.¹ It took 17 picoNewtons "or the weight of 17 red blood cells to stretch the MukBEF-associated DNA," says Bustamante. Upon releasing the DNA, it recondensed in discreet 35-nm steps, without the addition of energy or free MukBEF. The work suggests that MukBEF-MukBEF interactions trap DNA in a reversibly compact form, says Bustamante. Measuring the force of the MukBEF-DNA interaction is quite a feat, says Alex...