Final answer:
RecBCD protein is an enzyme that unwinds the DNA double helix at the replication fork and is involved in recombination of double-stranded DNA.
Step-by-step explanation:
The protein that is an enzyme encoded by three rec genes and is responsible for recombining double-stranded DNA is RecBCD. This enzyme acts as a helicase and unwinds the DNA double helix at the replication fork. It plays a crucial role in DNA replication and repair.