Part of the RFC clamp loader complex which loads the PCNA sliding clamp onto DNA. The complex possesses DNA-dependent ATPase activity which is further stimulated by PCNA. In conjunction with PCNA stimulates DNA synthesis by PolB, relieving inhibition by replication protein A (RPA). Heterohexamer composed of four small subunits (RfcS) and two large subunits (RfcL). Belongs to the activator 1 small subunits family. RfcS subfamily.