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Figure 1.
Fig. 1. The group I intron splicing reaction. (A) Secondary
structure of the pre-2S crystallization construct. The residues
discussed in the text are shown superimposed on the secondary
structure. RNA connectivity is depicted with a dashed line with
small arrows to show the 5' to 3' orientation. Exons are shown
in red. The coloring of other residues corresponds to the
structural element in which they are located: P4 to P6 (green),
P3 to P9 (blue), and J8/7 (purple). (B) Summary of the
biochemically defined ligands for active-site metal
coordination. The six oxygens shown in orange have been
implicated in metal-ion coordination on the basis of metal
specificity switch experiments (10-15), including four in the
substrates and two in the intron. Ligands biochemically shown to
coordinate the same metal are depicted with double-ended arrows.
The exon splicing reaction involving attack of the U-1 O3' on
the scissile phosphate with loss of the  G O3' is shown
with curved arrows. (C) Proposed three-metal-ion mechanism based
on differential Mn2+ affinity to sulfur/amino-substituted
substrates (21, 22). The four substrate ligands in (B) are
coordinated to three metal ions, M[A], M[B], and M[C].
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