Final answer:
DNA gyrase, a topoisomerase, is responsible for relieving tension caused by supercoiling ahead of the replication fork during DNA replication in E. coli.
Step-by-step explanation:
The enzyme in E. coli responsible for relieving the tension ahead of the replication fork, when DNA is unwound to form the replication bubble, is DNA gyrase, which is a type of topoisomerase. This enzyme helps in relaxing supercoiled DNA that results due to the unwinding of the double helix during replication. DNA gyrase achieves this by breaking and then rejoining the DNA strands which relieves the pressure caused by the supercoiling. This action allows the replication machinery to advance without the DNA strand being subjected to excessive torsional stress, preventing the DNA from breaking due to supercoiled tension.