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Journal of Virology, November 2000, p. 10612-10622, Vol. 74, No. 22
Basic Research Laboratory, Division of Basic
Sciences, National Cancer Institute, National Institutes of Health,
Bethesda, Maryland 20892
Received 18 April 2000/Accepted 14 August 2000
Bovine papillomavirus type 1 (BPV-1) late gene expression is
regulated at both transcriptional and posttranscriptional levels. Maturation of the capsid protein (L1) pre-mRNA requires a switch in 3'
splice site utilization. This switch involves activation of the
nucleotide (nt) 3605 3' splice site, which is utilized only in fully
differentiated keratinocytes during late stages of the virus life
cycle. Our previous studies of the mechanisms that regulate BPV-1
alternative splicing identified three cis-acting elements
between these two splice sites. Two purine-rich exonic splicing
enhancers, SE1 and SE2, are essential for preferential utilization of
the nt 3225 3' splice site at early stages of the virus life cycle.
Another cis-acting element, exonic splicing suppressor 1 (ESS1), represses use of the nt 3225 3' splice site. In the present
study, we investigated the late-stage-specific nt 3605 3' splice site
and showed that it has suboptimal features characterized by a
nonconsensus branch point sequence and a weak polypyrimidine track
with interspersed purines. In vitro and in vivo experiments showed that
utilization of the nt 3605 3' splice site was not affected by SE2,
which is intronically located with respect to the nt 3605 3' splice
site. The intronic location and sequence composition of SE2 are similar
to those of the adenovirus IIIa repressor element, which has been shown
to inhibit use of a downstream 3' splice site. Further studies
demonstrated that the nt 3605 3' splice site is controlled by a novel
exonic bipartite element consisting of an AC-rich exonic splicing
enhancer (SE4) and an exonic splicing suppressor (ESS2) with a UGGU
motif. Functionally, this newly identified bipartite element
resembles the bipartite element composed of SE1 and ESS1. SE4
also functions on a heterologous 3' splice site. In contrast, ESS2
functions as an exonic splicing suppressor only in a
3'-splice-site-specific and enhancer-specific manner. Our data indicate
that BPV-1 splicing regulation is very complex and is likely to be
controlled by multiple splicing factors during keratinocyte differentiation.
0022-538X/00/$04.00+0
Utilization of the Bovine Papillomavirus Type 1 Late-Stage-Specific Nucleotide 3605 3' Splice Site Is Modulated by
a Novel Exonic Bipartite Regulator but Not by an Intronic
Purine-Rich Element
*
Corresponding author. Present address for Zhi-Ming
Zheng: HAMB/DCS/NCI, Building 10, Room 13N240, 9000 Rockville Pike,
Bethesda, MD 20892. Phone: (301) 594-1382. Fax: (301) 480-8250. E-mail: zhengt{at}exchange.nih.gov. Mailing address for Carl C. Baker: BRL/DBS/NCI, Building 41, Room D804, 41 Library Dr. MSC 5055, Bethesda, MD 20892. Phone: (301) 496-2078. Fax: (301) 402-0055. E-mail:
ccb{at}nih.gov.
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