Ring ATPases that translocate disordered polymers possess lock-washer architectures that they impose on their substrates during transport via a hand-over-hand mechanism. Here, we investigate the operation of ring motors that transport substrates possessing a preexisting helical structure, such as the bacteriophage φ29 dsDNA packaging motor. During each cycle, this pentameric motor tracks one helix strand (the tracking strand), and alternates between two segregated phases: a dwell in which it exchanges ADP for ATP and a burst in which it packages a full turn of DNA in four steps. We challenge this motor with DNA-RNA hybrids and dsRNA substrates and find that it adapts the size of its burst to the corresponding shorter helical pitches by keeping three of its power strokes invariant while shortening the fourth. Intermittently, the motor loses grip when the tracking strand is RNA, indicating that it makes load-bearing contacts with the substrate that are optimal with dsDNA. The motor possesses weaker grip when ADP-bound at the end of the burst. To rationalize all these observations, we propose a helical inchworm translocation mechanism in which the motor increasingly adopts a lock-washer structure during the ATP loading dwell and successively regains its planar form with each power stroke during the burst.

中文翻译：

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DNA转移酶通过在平面和锁紧垫圈结构之间循环而起作用

易位无序聚合物的环ATP酶具有锁紧垫圈结构，它们在转运过程中通过移交机制强加在其底物上。在这里，我们研究了环形马达的操作，该马达运送具有预先存在的螺旋结构的底物，例如噬菌体φ29dsDNA包装马达。在每个循环中，此五聚体马达跟踪一个螺旋链（跟踪链），并在两个分离的阶段之间交替：一个将ADP交换为ATP的停顿期和一个将四个步骤包装完整DNA的爆发期。我们用DNA-RNA杂种和dsRNA底物挑战该电机，发现它通过使其三个动力冲程保持不变而缩短了第四个，使其爆发的大小适应相应的较短螺旋螺距。间歇地 当跟踪链为RNA时，电机失去抓地力，这表明它与dsDNA最佳的底物进行了承重接触。爆发结束时绑定ADP时，电动机的抓地力较弱。为了使所有这些观察合理化，我们提出了一种螺旋形的蠕虫移位机制，其中电动机在ATP加载停留期间逐渐采用锁紧垫圈结构，并在爆发期间的每个功率冲程中逐渐恢复其平面形式。