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. Heteropentamer composed of four small subunits (RfcS) and one large subunit (RfcL). Both subunits interact with PCNA. Belongs to the activator 1 small subunits family. RfcS subfamily.