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Mol. Cell. Biol., 12 1997, 7099-7107, Vol 17, No. 12
DY Hwang and JB Cohen
Both experimental work and surveys of the lengths of internal exons in
nature have suggested that vertebrate internal exons require a minimum size
of approximately 50 nucleotides for efficient inclusion in mature mRNA.
This phenomenon has been ascribed to steric interference between complexes
involved in recognition of the splicing signals at the two ends of short
internal exons. To determine whether U1 small nuclear ribonucleoprotein, a
multicomponent splicing factor that is involved in the first recognition of
splice sites, contributes to the lower size limit of vertebrate internal
exons, we have taken advantage of our previous observation that U1 small
nuclear RNAs (snRNAs) which bind upstream or downstream of the 5' splice
site (5'SS) stimulate splicing of the upstream intron. By varying the
position of U1 binding relative to the 3'SS, we show that U1-dependent
splicing of the upstream intron becomes inefficient when U1 is positioned
48 nucleotides or less downstream of the 3'SS, suggesting a minimal
distance between U1 and the 3'SS of approximately 50 nucleotides. This
distance corresponds well to the suggested minimum size of internal exons.
The results of experiments in which the 3'SS region of the reporter was
duplicated suggest an optimal distance of greater than 72 nucleotides. We
have also found that inclusion of a 24-nucleotide miniexon is promoted by
the binding of U1 to the downstream intron but not by binding to the 5'SS.
Our results are discussed in the context of models to explain constitutive
splicing of small exons in nature.
Copyright © 1997, American Society for Microbiology
U1 small nuclear RNA-promoted exon selection requires a minimal distance between the position of U1 binding and the 3' splice site across the exon
Department of Molecular Genetics and Biochemistry, University of Pittsburgh School of Medicine, Pennsylvania 15261, USA.
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