The U40C mutation abrogates L30 binding. A. Schematic illustrating the binding sites of SBP2  and L30 (this work) on the SECIS, as determined by RNA footprinting. The position of the U40C point mutation is indicated. B. UV cross-linking experiments were performed using the 32P-labeled wild-type PHGPx SECIS or the U40C mutant RNA, which were incubated with increasing amounts of purified L30 as indicated. After RNase digestion, the products were analyzed by SDS-PAGE and autoradiography.