力在细胞的很多重要生理过程（例如细胞运动、信号转导、免疫反应甚至基因的维护等等）发挥重要调控作用。力的重要性在力学生物学中得到了充分的重视。但在细胞内，特别是在分子水平，力起重要作用的直接证据还不多。我们在前期的初步研究中发现，力能促进一个叫Pif1的解旋酶的持续解旋能力。这使得我们联想到最近的一个重要的但无法解释的发现：本来解旋能力不高的Pif1却在断裂诱导的DNA复制过程（BIR）中要解开很长的DNA。我们认为BIR过程所产生的D-loop (D-环)结构中的力是调制这一生理过程的重要因素。本项目中我们利用高精度单分子技术（磁镊和FRET），以碱基分辨能力研究Pif1解旋酶的细致解旋动力学过程，探讨Pif1解旋的分子机理以及力增强Pif1解旋活性的机制。我们还将在体外单分子水平模拟BIR过程，验证我们提出的重要假设，阐述Pif1在BIR中起重要作用的分子基础。

Force is an important regulation parameter often adopted by cells in many life processes such as cell motion, signal transduction and genome maintenance, to name just a few. The effects of force have long been the subjects of Mechanobiology. But direct evidences of them are still very rare in the DNA metabolism at the molecular level. In our recent research on a DNA helicase called Pif1, we found that the enzymatic activity of Pif1 is strongly enhanced by the force in our magnetic tweezers assays. We therefore assumed that cell uses this enhancement effect to regulate the activity of Pif1 in a very important DNA repair pathway called the DNA break-induced replication (BIR) because the D-loop structure in the BIR creates a strong tension on DNA. In this project we will study the effect of force on the activity of DNA helicases, the relevant molecular mechanism and the potential regulation functions. We will use single molecule techniques to study how the force affects the activity of the Pif1 and try to understand the relevant molecular mechanism. We will also mimic the BIR process in vitro to demonstrate our assumption and clarify the role of Pif1 in this processs.