5e SRP RNA binding activities of mutated SRP72 fragments. For each construct, 82.5 ng of in vitro transcribed 5e SRP RNA was incubated with purified protein at the concentrations 0.056, 0.18, 0.56, and 1.8 uM. Samples were incubated with Ni-NTA magnetic agarose beads as described in the Methods. The bound protein and RNA were analyzed by SDS PAGE followed by staining with Coomassie blue (labeled p below each panel) and Ethidium bromide. The amount of bound RNA was measured and normalized to 100 percent in reference to the binding observed with 72 h, 72 j, or 72 k. The insert on the bottom right depicts the design of the assay.