Mechanism of helicase translocation along nucleic acid

In cells, helicase translocation along nucleic acid is essential for many biological processes. However, so far, the mechanism of this translocation is not fully understood. Recent studies show that helicase might translocate through two processes, active process and passive process, with different translocation rate. In this study, a mechanochemical model including such two processes is presented for ribosome translocation along messenger RNA during translation. In which, each of the two processes consists of two sub-processes, translocation factor binding sub-process and the following mechanochemical sub-process in which ribosome makes a forward translocation step. Ribosome switches stochastically between these two processes with external force dependent rates. By this model, we found that, with the increase of external force, the mean translocation rate of ribosome increases from one lower limit to another upper limit, and both of these two limits increase with concentrations of the translocation factors. Under high external force, ribosome translocates mainly through active process. At saturating concentration of translocation factors, the translocation is limited by mechanochemical sub-processes.