C-RPS3 (94–244 aa), but not N-RPS3 (1–94 aa), contributes to RECQL4 helicase activity inhibition. All helicase reactions contain 50 nm RECQL4 protein and 20 nm 5′FAM-labeled 3′ overhang substrate. Substrate and product structures are illustrated on the right side of the gels. * represents position of FAM label on the substrate. A, representative PAGE showing the RECQL4 helicase assay performed in the presence or absence of indicated concentrations of full-length FL-RPS3 (1–244 aa). B, representative PAGE showing the RECQL4 helicase assay performed in the presence or absence of indicated concentrations of C-RPS3 (94–244 aa). C, representative PAGE showing RECQL4 helicase assay performed in the presence or absence of indicated concentrations of N-RPS3 (1–94 aa). D, graph depicting percentage unwinding by RECQL4 plotted against final concentration of FL-RPS3, N-RPS3, or C-RPS3 in RECQL4 helicase reaction. The data points shown are the means of triplicate experiments with S.D. indicated as error bars.