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Figure 8.
Figure 8 Proposed model of the RNA chaperone activity of
nucleolin for proper folding of the 5' ETS region between
nucleotides 1671 and 3549 of human 47S pre-rRNA. A schematic
representation of the predicted secondary structure of this
region in the mature pre-rRNA based on phylogeny (Renalier et
al., 1989) and electron microscopy (Wellauer et al., 1974;
Schibler et al., 1975) studies is shown on the right. The
putative NRE binding sites in this sequence are indicated by
black rectangles. They are all found in double-stranded regions
of the mature pre-rRNA, so nucleolin (indicated by the black
oval ring) is not expected to be bound. On the left side of the
figure are shown schematically two alternate structures that the
RNA can adopt with (top) or without (bottom) nucleolin. Without
nucleolin, the RNA can be kinetically trapped in alternative
stable structures, which have to unfold to form the mature
pre-rRNA, with the result that formation of the mature pre-rRNA
will be slow. The bound nucleolin promotes and/or stabilizes
stem–loops at the NRE consensus sites, preventing the
formation of alternative stable helices, and then dissociates to
allow the final structure to form.
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