SANTA CLARA The 49ers brought in free-agent safety Donte Whitner to fill a leadership vacuum in their secondary, and the five-year veteran isn't shying away from his assignment.
"The front seven is set right now, from what I've seen," said Whitner, who went through his first practice Saturday. "We have the 'backers. We have the defensive line. What we're really going to have to shore up is the secondary. And I take responsibility for that."
The heart of the 49ers' defense in recent seasons has been its linemen and inside linebackers. That group has been excellent at defending the run, finishing sixth in that category last season.
Defending the pass has been more problematic. The team finished 24th in passing yards allowed in 2010 and gave up 25 touchdowns through the air.
Three of the 49ers' free-agent signings Whitner, starting cornerback Carlos Rogers and safety Madieu Williams were made to solidify the defensive backfield.
Whitner, 26, also shed some light on the type of defense new coordinator Vic Fangio intends to run this season.
He said he expected the 49ers' defense to be as aggressive as the 3-4 defenses the Steelers and Packers operate. Whitner had 140 tackles last season for Buffalo, fifth best in the league and the most for a non-linebacker.
"I'm looking forward to being able to do some things as far as disguise and blitz really fool with quarterbacks," he said. "And the coaches here will allow me to do that."
Live from Santa Clara The 49ers have been in full pads every practice but one for the past seven days. Saturday's session featured "live" tackling, the highlight of which came when running back Anthony Dixon tried to break wide to his left on a goal-line handoff.
He was run down by Patrick Willis, who not only made the tackle but stripped Dixon of the ball. It was scooped up by Ray McDonald, who found only one person between him and a 95-yard return coach Jim Harbaugh, who after a moment's hesitation, signaled to McDonald to keep running downfield.
Et cetera Rookie receiver Chris Hogan, who had a solid start to training camp, was in a walking boot Saturday, the result of an injury he sustained while making a 50-yard sideline catch Friday.
Harbaugh said Jeremiah Masoli, whom the team initially signed to play running back, would remain at quarterback. Masoli worked with the third-team offense Saturday.
The highlight of Saturday's two-minute drill: A 50-yard touchdown pass from Alex Smith to receiver Ted Ginn. Smith badly underthrew Ginn on a similar play Friday.