Patterns of cleavages induced by lead ions in defined RNA secondary structure motifs

We have characterized the susceptibility of various RNA bulges, loops and other single-stranded sequences to hydrolysis promoted by Pb2+. The reactivity of bulges depends primarily on the structural context of the flanking base-pairs and the effect of nucleotide present at the 5' side of the bulge is particularly strong. The efficiency of stacking interactions between the bulged residue and its neighbors seems to determine cleavage specificity and efficiency. Hydrolysis of two-and three-nucleotide bulges depends only slightly on their nucleotide composition. In the case of terminal loops, the efficiency of their hydrolysis usually increases with the loop size and strongly depends on its nucleotide composition. Stable tetraloops UUCG, CUUG and GCAA are resistant to hydrolysis, while in some other loops of the GNRA family a single, weak cleavage occurs, suggesting the existence of structural subclasses within the family. A very efficient, specific hydrolysis of a phosphodiester bond in the single-stranded region adjacent to the stem in oligomer 12 resembles highly specific cleavages of some tRNA molecules. The reaction occurs in the presence of Pb2+, but not in the presence of several other metal ions. The Pb2+-cleavable RNA domain may be considered another example of lead-zyme. The results of Pb2+-induced hydrolysis in model RNA oligomers should be useful in interpretation of cleavage patterns of much larger, naturally occurring RNA molecules. (C) 1998 Academic Press Limited.