Site-specific cross-linking of SXL to 5-IU containing tra NSS (A) or AdML (B) Py tracts. Each of the 5′-end-labeled RNAs containing either no 5-IU (None) or a single 5-IU at various positions was cross-linked to SXL(W), SXL(W)ΔC, and SXL(W)ΔNΔC. The cross-linked protein was cleaved with NCS and resolved in an SDS–polyacrylamide gel. The positions of the 5-IU used for the cross-linking are shown below, and the percentage relative cross-linking to either RRM1 or RRM2 is shown above the autoradiograms; (–) weak cross-linking. For reference, the tra (NSS) and the AdML Py-tract sequences are shown. Lanes a, b, and c correspond to various SXL derivatives. The positions of the uncleaved protein (curly brackets) and of the cleaved fragments corresponding to RRM1 and RRM2 (arrows) are shown. (C) Summary of the X-ray structure of SXL and the NSS Py tract of tra (Handa et al. 1999). The boxes refer to RRM1 and RRM2, and lines below the sequence refer to the nucleotides that were either degraded or did not contact SXL in the X-ray structure.
