Bates A D, Maxwell A
Department of Biochemistry, University of Leicester, UK.
EMBO J. 1989 Jun;8(6):1861-6. doi: 10.1002/j.1460-2075.1989.tb03582.x.
DNA gyrase introduces negative supercoils into closed-circular DNA using the free energy of ATP hydrolysis. Consideration of steric and thermodynamic aspects of the supercoiling reaction indicates that there should be a lower limit to the size of DNA circle which can be supercoiled by gyrase. We have investigated the supercoiling reaction of circles from 116-427 base pairs (bp) in size and have determined that gyrase can supercoil certain relaxed isomers of circles as small as 174 bp, dependent on the final superhelix density of the supercoiled product. Furthermore, this limiting superhelical density (-0.11) is the same as that determined for the supercoiling of plasmid pBR322. We also find that although circles in the range 116-152 bp cannot be supercoiled, they can nevertheless be relaxed by gyrase when positively supercoiled. These data suggest that the conformational changes associated with the supercoiling reaction can be carried out by gyrase in a circle as small as 116 bp. We discuss these results with respect to the thermodynamics of DNA supercoiling and steric aspects of the gyrase mechanism.
DNA促旋酶利用ATP水解的自由能将负超螺旋引入闭环DNA。对超螺旋反应的空间和热力学方面的考虑表明，能够被促旋酶超螺旋化的DNA环的大小应该存在一个下限。我们研究了大小在116 - 427个碱基对（bp）的环的超螺旋反应，并确定促旋酶能够使某些小至174 bp的环状松弛异构体超螺旋化，这取决于超螺旋产物的最终超螺旋密度。此外，这个极限超螺旋密度（-0.11）与质粒pBR322超螺旋化所确定的密度相同。我们还发现，虽然116 - 152 bp范围内的环不能被超螺旋化，但当它们处于正超螺旋状态时，促旋酶仍能使其松弛。这些数据表明，与超螺旋反应相关的构象变化可以由促旋酶在小至116 bp的环中进行。我们结合DNA超螺旋的热力学和促旋酶机制的空间方面来讨论这些结果。
