Question

Which of the following promotes branch migration at higher rates than does Rec-A?

• A. Rec-B
• B. Rec-C
• C. Rec-D
• D. Ruv-A and Ruv-B

Hint:

Ruv-A and Ruv-B proteins are key to promoting efficient branch migration during recombination, allowing the recombination process to proceed at higher rates.

Answer 1

The Correct Option is D

Step 1: Understanding the proteins involved in branch migration.
Rec-A is a protein involved in homologous recombination, promoting the strand invasion process. However, Ruv-A and Ruv-B proteins are involved in branch migration during recombination, enhancing the rate of migration compared to Rec-A.
Step 2: Analyzing the options.
(1) Rec-B: This is incorrect as Rec-B is part of the RecBCD complex that initiates recombination but is not as effective in branch migration as Ruv-A and Ruv-B.
(2) Rec-C: Rec-C is also involved in DNA recombination but does not enhance branch migration more than Rec-A.
(3) Rec-D: Rec-D is another subunit of RecBCD, but it does not promote branch migration more than Rec-A.
(4) Ruv-A and Ruv-B: Correct — Ruv-A and Ruv-B are directly involved in branch migration during homologous recombination and promote migration at higher rates than Rec-A.
Step 3: Conclusion.
The correct answer is (4) Ruv-A and Ruv-B, as they promote branch migration more effectively than Rec-A.

Answer 2

Branch migration is the process of extending a Holliday junction along a DNA duplex during homologous recombination, and different proteins in the recombination pathway play different roles in this step. Testing each option:

1. Rec-B: Rec-B is a subunit of the RecBCD helicase-nuclease complex, whose job is to unwind and resect DNA ends to generate a single strand for RecA to load onto -- it acts upstream of branch migration, not as the enzyme that drives migration itself.
2. Rec-C: Rec-C is also part of the RecBCD complex, contributing to recognition of the recombination hotspot sequence; like Rec-B, its role is in the earlier end-processing stage, not in driving branch migration.
3. Rec-D: Rec-D is the third subunit of RecBCD, providing helicase activity for unwinding DNA during end resection, again functioning before the branch migration stage.
4. Ruv-A and Ruv-B: RuvA recognizes and binds the Holliday junction structure, and RuvB is an ATP-driven motor protein that RuvA recruits to the junction; together they actively pump the junction along the DNA duplex, driving branch migration at a considerably faster rate than RecA can achieve on its own.

Since the RecBCD subunits act at the end-resection stage rather than at the migration step, and RuvAB is the dedicated, ATP-powered branch migration motor, RuvAB is the complex responsible for the faster migration rate.

Therefore, the correct answer is Ruv-A and Ruv-B.