Part of the RFC clamp loader complex which loads the PCNA sliding clamp onto DNA. The complex possesses DNA-dependent ATPase activity. Heteropentamer composed of four small subunits (RfcS) and one large subunit (RfcL). A homotetramer of this subunit interacts with PCNA heterodimer PCNA1-PCNA2. Belongs to the activator 1 small subunits family. RfcS subfamily.