ARLINGTON, TX - JUNE 22: Brett Wallace #29 of the Houston Astros catch the ball for the out during the game against the Houston Astros at Rangers Ballpark in Arlington on June 22, 2011 in Arlington, Texas. (Photo by Rick Yeatts/Getty Images)
Wallace and Johnson had been Houston's regular options at first base and third base, respectively, but both have gradually lost playing time due to poor performance. Since May 4, Wallace is hitting just .220/.302/.305 with 2 homers in 73 games, while Johnson has pretty much struggled all season. In order to fill their roster spots, Houston called up outfielder Brian Bogusevic and infielder Jimmy Paredes.
In the short-term, it appears that the Astros are moving Carlos Lee from left field to first base in order to fill Wallace's spot, and Paredes is taking over at third base for Johnson. I wouldn't expect Paredes to last very long in Houston, though; the 22-year-old has hit just .266/.296/.414 in Double-A this season, and clearly still needs to work on his hitting in the minors.