The Role of LigD in Non- Homologous End-Joining in Mycobacterium tuberculosis

Abstract

Repairing damaged DNA is necessary for the survival and accurate reproduction of organisms. To this end, numerous DNA damage repair pathways have evolved. Non-homologous end-joining is a repair pathway that repairs double-stranded breaks in DNA, without using a homologous sequence as a template. In bacteria, the two proteins necessary to facilitate non-homologous end joining are Ku and LigD, a DNA ligase. This project investigated how six different point mutations on LigD affected the DNA ligation activity. Both bulk and single molecule methods were utilized. For bulk assays, electrophoretic mobility assays, pulldown assays, and ligation assays were used to study how well Ku and LigD bind to and ligate DNA, and how the mutations affect this. The ligation activity was also investigated using nanofluidics, visualizing the DNA on a single molecule level. From these assays, it was shown that three of the point mutations (R198E, D162R, V194D) resulted in lowered ligation activity compared to wild type LigD. One of the mutations (K579E) increased the ligation activity. The mutation L580E caused DNA fragmentation, and for LigD D522R, the results from the assays were contradictory.