Structure of M1 RNA as determined by psoralen cross-linking.

The RNA moiety of ribonuclease P from Escherichia coli (M1 RNA) has been photoreacted with 4'-(hydroxymethyl)-4,5',8-trimethylpsoralen (HMT) and long-wave UV light (320-380 nm) in a buffer containing 60 mM Mg2+, where the RNA moiety acts as a true catalyst of tRNA processing. Limited specific digestion and two-dimensional gel electrophoresis yield fragments cross-linked by HMT. By photoreversal of the isolated cross-linked fragments and enzymatic sequencing of the fragments, the positions of the cross-links have been elucidated. This method allows us to locate the cross-link to +/- 15 nucleotides. Further assignments of the exact locations of the cross-links have been made on the basis of the known photoreactivity of the psoralen with different bases. Nine unique cross-links have been isolated in the M1 RNA including four long-range interactions. The short-range interactions are discussed here in detail.