Question

Researchers characterized a human protein with a role in Holliday junction resolution. They purified and used the protein in an assay with the DNA substrate shown below on the left. Each of the four strands of DNA (A, B, C, D) is 60 nucleotides (nt) long, and only the DNA strand marked A is end-labeled with 50-32P. The researchers set up three reactions with the DNA substrate, incubating (1) with no protein, (2) withthepurifiedprotein(ProteinX),and(3)withRecA(asacontrol).Theproductsfromeachreactionwereruninseparatelanes of adenaturingpolyacrylamidegel.An autoradiogramof thegel is shown below on the right.

A. Given these data, propose a function for Protein X.
B. Based on your answer in part A, this protein functions similarly to what E. coli protein?

Answer 1

Protein X functions as an endonuclease, cleaving DNA at specific sites. It is similar to the E. coli protein, RecA, which is involved in DNA repair and recombination.

Step-by-step explanation:

The function of Protein X can be deduced from the autoradiogram of the gel. In the absence of Protein X, the DNA substrate remains intact and appears as a single band. However, in the presence of Protein X, multiple bands are observed, indicating that the protein is causing the DNA to be cleaved or cut into smaller fragments. This suggests that Protein X functions as an endonuclease, an enzyme that cleaves DNA at specific sites.

Based on this function, Protein X functions similarly to the E. coli protein called RecA. RecA is a multifunctional protein involved in DNA repair and recombination. It also catalyzes the formation and resolution of Holliday junctions, which are key intermediates in genetic recombination.