Figure 4.

Mer1p activates splicing of introns with BPS mutations. Primer extension analysis of splicing of various AMA1 reporter mRNAs isolated from yeast containing a MER1 expression plasmid (+ pMER1) or a control plasmid (–). Five micrograms of total RNA were used in each reaction. The –BP constructs contain the C to U mutation in the third position of the BPS. The NT7-15 construct is an AMA1 intron with alterations that disrupt the splicing silencer at nucleotides 7–15 of the intron; it does not require Mer1p for efficient splicing. The relative positions of the splicing silencer (S) and enhancer (E) with respect to the 5′ splice site and BPS are indicated above the gel phosphorimage. Splicing efficiencies were averaged for several replicates and are reported below each lane. U and S refer to cDNAs from unspliced and spliced RNAs, respectively.