(A) HeLa cells were transfected with plasmids expressing the same pre-mRNAs as those assayed in except that the 3′ splice site was wild type. Splicing was assayed by semi-quantitative RT-PCR; products generated from use of the proximal or distal 5′ splice sites are indicated. Lane 1, Motif D; lane 2, Motif B; lane 3, Motif C; lane 4, Motif A; lane 5, no silencer; lane 6, Motif F; lane 7, Motif E; Lane 8, pre-mRNA lacking a silencer in which the proximal 5′ splice site was inactivated by mutation as in .
(B) In vivo splicing of pre-mRNAs identical to those in (A) except that the silencing motif was inserted at positions +11 to +22 (intronic) or −18 to −7 (exonic) relative to the distal 5′ splice site as appropriate; thus each silencing motif was present twice in each pre-mRNA at the same position relative to both the proximal and distal 5′ splice sites. Lane designations are as in (A).
(C) In vivo splicing of pre-mRNAs with duplicated weakened 5′ splice sites (see text). As in (A) each pre-mRNA contained one specific silencing motif upstream or downstream of the proximal 5′ splice site. Lane designations are as in (A).
(D) In vivo splicing of pre-mRNAs with duplicated weakened 5′ splice sites with duplicated silencing motifs. As in (B), the specific silencer motifs were inserted upstream or downstream as appropriate of the distal 5′ splice site when the same motif was upstream or downstream of the proximal 5′ splice site. Lane designations are as in (A).
(E) Body-labeled dual 5′ splice site pre-mRNAs identical to those assayed for splicing in vivo in (A) and (B) except that the 3′ splice site was inactivated by mutation, were spliced in vitro. Each pre-mRNA contained the specific silencing motifs described in the Legend to either upstream or downstream of the proximal 5′ splice site. Lane 1, Motif D; lane 2, Motif B; lane 3, Motif C; lane 4, Motif A; lane 5, no silencer motif; lane 6, Motif F; lane 7, Motif E. For lanes 8–14, specific silencer motifs were inserted either upstream or downstream of the distal 5′ splice site as appropriate such that each pre-mRNA contained identical silencer motifs at the same position relative to both 5′ splice sites. Lane 8, Motifs D; lane 9, Motifs B; lane 10, Motifs C; lane 11, Motifs A; lane 12, no silencers; lane 13, Motifs F; lane 14, Motifs E.
(F) In vitro splicing of pre-mRNAs lacking the distal 5′ splice site in the presence or absence of silencing elements. The same pre-mRNAs as in (E), lanes 1–7, lacking the distal 5′ splice site with a wild type 3′ splice site and with the specific silencer elements present upstream or downstream of the remaining 5′ splice site were spliced in vitro. Lane 1, Motif D; lane 2, Motif B; lane 3, Motif A; lane 5, no silencer; lane 6, Motif F; lane 7, Motif E.