We consider the sum rate maximization problem in downlink OFDMA systems subject to total transmit power and minimum user rate constraints. A low complexity suboptimal resource allocation algorithm is proposed for joint optimization of multiuser subcarrier assignment and power allocation. The algorithm is based on Lagrangian relaxation where the non-convex optimization problem is decomposed into a master problem and a subproblem. Subproblem has a similar form as standard weighted sum rate maximization subject to total transmit power. Standard subgradient method is used in the master problem and a suboptimal algorithm inspired from primal decomposition techniques is also proposed to approximately solve the subproblem. Simulations are provided to compare the performance of the proposed algorithm to fixed spectrum assignment based algorithms with adaptive power control.