Johnson is notable for his time spend with Harbaugh at the University of San Diego. Johnson earned multiple career passing records at the school, including touchdown passes (113), completions (724) and passing yards (9,699). Johnson was drafted in the fifth round back in 2008 and spent time with the Tampa Bay Buccaneers.
He also spent time with the Cleveland Browns and spent last season with the Cincinnati Bengals. He did have a short stint with the 49ers, in which he lost the job for third-string quarterback Scott Tolzien (despite out-playing Tolzien in the preseason finale). With his addition, the 49ers now have Colin Kaepernick, Blaine Gabbert, McLeod Bethel-Thompson, Kory Faulkner and now Johnson.
Johnson was released just a few days ago. Tupou was originally signed to a reserve/future contract in January
At this point, rotating quarterbacks behind Kaepernick is just the reality of the 49ers. Tolzien isn't here any longer, nor is B.J. Daniels, who originally beat him out for the third-string job. Daniels' replacement, John Skelton, is also no longer with the team. This is how Jim Harbaugh done things, so I'm not going to even venture a guess as to what's going to happen. Johnson plays a lot like Kapernick, for what it's worth. But that might mean literally nothing at this point.