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Mol. Cell. Biol., Jun 1996, 2579-2584, Vol 16, No. 6
C Cooke and JC Alwine
The 5' cap of a mammalian pre-mRNA has been shown to interact with splicing
components at the adjacent 5' splice site for processing of the first exon
and the removal of the first intron (E. Izaurralde, J. Lewis, C. McGuigan,
M. Jankowska, E. Darzynkiewicz, and I.W. Mattaj, Cell 78:657-668, 1994).
Likewise, it has been shown that processing of the last exon and removal of
the last intron involve interaction between splicing components at the 3'
splice site and the polyadenylation complex at the polyadenylation signal
(M. Niwa, S. D. Rose, and S.M. Berget, Genes Dev. 4:1552-1559, 1990; M.
Niwa and S. M. Berget, Genes Dev. 5:2086-2095, 1991). These findings
suggest that the cap provides a function in first exon processing which is
similar to the function of the 3' splice site at last exon processing. To
determine whether caps and 3' splice sites function similarly, we compared
the effects of the cap and the 3' splice site on the in vitro utilization
of the simian virus 40 late polyadenylation signal. We show that the
presence of a m7GpppG cap, but not a cap analog, can positively affect the
efficiency of polyadenylation of a polyadenylation-only substrate. Cap
analogs do not stimulate polyadenylation because they fail to bind
titratable cap-binding factors. The failure of cap analogs to stimulate
polyadenylation can be overcome if a 3' splice site is present upstream of
the polyadenylation signal. These data indicate that factors interacting
with the cap or the 3' splice site function similarly to affect
polyadenylation signal, along with m7GpppG cap, is inhibitory to
polyadenylation. This finding suggests that the interaction between the
cap-binding complexes and splicing components at the 5' splice site may
form a complex which is inhibitory to further processing if splicing of an
adjacent intron is not achieved.
Copyright © 1996, American Society for Microbiology
The cap and the 3' splice site similarly affect polyadenylation efficiency
Graduate Group of Molecular Biology, University of Pennsylvania, Philadelphia 19104-6142, USA.
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