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Molecular and Cellular Biology, January 1999, p. 364-375, Vol. 19, No. 1
Molecular Microbiology and Immunology, School
of Medicine, University of Missouri
Received 18 June 1998/Returned for modification 18 August
1998/Accepted 23 September 1998
The alternatively spliced 290-nucleotide NS2-specific exon of the
parvovirus minute virus of mice (MVM), which is flanked by a large
intron upstream and a small intron downstream, constitutively appears
both in the R1 mRNA as part of a large 5'-terminal exon (where it is
translated in open reading frame 3 [ORF3]), and in the R2 mRNA as an
internal exon (where it is translated in ORF2). We have identified a
novel bipartite exon enhancer element, composed of CA-rich and
purine-rich elements within the 5' and 3' regions of the exon,
respectively, that is required to include NS2-specific exon sequences
in mature spliced mRNA in vivo. These two compositionally different
enhancer elements are somewhat redundant in function: either element
alone can at least partially support exon inclusion. They are also
interchangeable: either element can function at either position. Either
a strong 3' splice site upstream (i.e., the exon 5' terminus) or a
strong 5' splice site downstream (i.e., the exon 3' terminus) is
sufficient to prevent skipping of the NS2-specific exon, and a
functional upstream 3' splice site is required for inclusion of the
NS2-specific exon as an internal exon into the mature, doubly spliced
R2 mRNA. The bipartite enhancer functionally strengthens these termini:
the requirement for both the CA-rich and purine-rich elements can be
overcome by improvements to the polypyrimidine tract of the upstream
intron 3' splice site, and the purine-rich element also supports exon
inclusion mediated through the downstream 5' splice sites. In summary,
a suboptimal large-intron polypyrimidine tract, sequences within the
downstream small intron, and a novel bipartite exonic enhancer operate
together to yield the balanced levels of R1 and R2 observed in vivo. We suggest that the unusual bipartite exonic enhancer functions to mediate
proper levels of inclusion of the NS2-specific exon in both singly
spliced R1 and doubly spliced R2.
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
CA- and Purine-Rich Elements Form a Novel Bipartite
Exon Enhancer Which Governs Inclusion of the Minute Virus of Mice
NS2-Specific Exon in Both Singly and Doubly Spliced mRNAs
Columbia, Columbia, Missouri
65212
*
Corresponding author. Mailing address: Molecular
Microbiology and Immunology, School of Medicine, University of
Missouri
Columbia, Columbia, MO 65212. Phone: (573) 882-3920. Fax:
(573) 882-4287. E-mail: pinteld{at}missouri.edu.
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