Metal binding and base ionization in the U6 RNA intramolecular stem-loop structure.

U6 RNA is a key component of the catalytic core of the spliceosome. A metal ion essential for the first catalytic step of pre-mRNA splicing binds to the U80 Sp phosphate oxygen within the yeast U6 intramolecular stem-loop (ISL). Here we present the first structural data for U6 RNA, revealing the three-dimensional structure of the highly conserved U6 ISL. The ISL binds metal ion at the U80 site with the same stereo specificity as the intact spliceosome. The metal-binding site is adjacent to a readily protonated C.A wobble pair. Protonation of the C.A pair and metal binding are mutually antagonistic. These results support a ribozyme model for U6 RNA function and suggest a possible mechanism for the regulation of RNA splicing.