When it comes to corralling DNA, MukB is the little condensin that could, say Yuanbo Cui, Aoya Petrushenko, and Valentin Rybenkov (University of Oklahoma, Norman, OK).
Small in size but with a big effect, condensins are believed to stabilize large DNA loops, thereby giving order to an otherwise tangled mess of chromatin. To investigate the mechanics of the E. coli condensin MukB, the authors stretched DNA between a glass capillary and a magnetic bead and monitored MukB-induced DNA condensation.
The DNA compacted in a series of discrete steps, indicating that multiple MukBs are at work. The process could be reversed by applying excess force in the opposite direction, but the longer a condensed complex was allowed to sit, the more force was required, suggesting that MukBs link up over time.
A small decrease in MukB concentration led to a dramatic decline in condensation rate. The slowed rate was primarily due to a longer lag before condensation began, which indicates that multiple MukBs must first come together in a nucleation step. ATP stimulated this nucleation but had no subsequent effect, and faster hydrolysis did not lead to faster nucleation. ATP thus seems to be a structural, rather than energetic, component.
According to Rybenkov, MukBs are acting as clamps to hold loops together. “I think we now have a biochemical explanation for how stable loops appear in such a big unruly molecule as DNA,” he says.