When ram is given a rating of 1600mhz, it means it has been tested designed to run at a MAXIUMUM of 1600mhz.
This is analogous to fans. A fan can be designed to run at 2200rpm,but it can run at lower speeds, such as 1600rpm.
Therefore, a 1600mhz can easily run at 1333mhz, since it is essentially, like a fan that is spinning slower then it can spin.
When you install your ram, you mobo will automatically clock the ram at 1333mhz, with no problems.