The bacterial condensin MukBEF compacts DNA into a repetitive, stable structure

摘要

Condensins are conserved proteins containing SMC ( structural maintenance of chromosomes) moieties that organize and compact chromosomes in an unknown mechanism essential for faithful chromosome partitioning. We show that MukBEF, the condensin in Escherichia coli, cooperatively compacts a single DNA molecule into a. lament with an ordered, repetitive structure in an adenosine triphosphate (ATP) binding-dependent manner. When stretched to a tension of similar to17 pico-newtons, the. lament extended in a series of repetitive transitions in a broad distribution centered on 45 nanometers. A. lament so extended and held at a lower force recondensed in steps of 35 nanometers or its multiples; this cycle was repeatable even in the absence of ATP and free MukBEF. Remarkably, the pattern of transitions displayed by a given. lament during the initial extension was identical in every subsequent extension. Hence, after being deformed micrometers in length, each. lament returned to its original compact structure without the addition of energy. Incubation with topoisomerase I increased the rate of recondensation and allowed the structure to extend and reform almost reversibly, indicating that supercoiled DNA is trapped in the condensed structure. We suggest a new model for how MukBEF organizes the bacterial chromosome in vivo.