Write a question about the following article: Coming off their divisional home win over the Browns, the Steelers flew to Candlestick Park for a Week 16 interconference duel with the San Francisco 49ers on Monday night. Pittsburgh trailed in the first half with 49ers kicker David Akers getting a 22-yard field goal in the first quarter, followed by a 38-yard field goal in the second quarter. The Steelers would answer in the third quarter with a 51-yard field goal from kicker Shaun Suisham, but San Francisco came right back with quarterback Alex Smith completing a 1-yard touchdown pass to tight end Vernon Davis. The 49ers would pull away in the fourth quarter with running back Frank Gore getting a 5-yard touchdown run.

Question about the article:
What was the longest touchdown run of the game?