DNA ligases are used in both DNA replication and repair to seal breaks in the DNA. But DNA damage can result in single- or double-strand breaks that are not normal ligase substrates. These need to be processed first before a ligase can act on them. One of the enzymes that is recruited to some of such breaks is called PNK. It has two separate activities on the DNA, both of which can help provide a canonical ligase substrate. Which of the following activities would you expect PNK to have in this context?
A) 5′ kinase (phosphorylation of a free 5′-OH group) and 3′ kinase
B) 5′ phosphatase (dephosphorylation to create a free 5′-OH group) and 3′ phosphatase
C) 3′ kinase and 3′ phosphatase
D) 5′ phosphatase and 3′ kinase
E) 5′ kinase and 3′ phosphatase

Question

Quizplus · Accepted Answer

The DNA ligase reaction normally requires a free 3′-OH group and a 5′-phosphate group on the two DNA ends to proceed. If a break has created 3′-phosphate and 5′-OH ends instead, for example, an enzyme such as PNK can dephosphorylate the 3′ end in the upstream molecule, and phosphorylate the 5′ end in the downstream molecule, to create the canonical ends.

DNA Ligases Are Used in Both DNA Replication and Repair