Preparation of the mutagenized libraries of the S. cerevisiae PRP8 gene. (A) Alignment of the carboxy-terminal segment of Prp8 from S. cerevisiae (EMBL Z24732) and Homo sapiens (GenBank accession no. AF092565). The carboxy-terminal segment of yPrp8 (5506–6195 nucleotides, amino acids 1836–2065) was used in mutagenic PCR to create a random library. In addition, two pairs of amino acids (*) flanking the mapped site of the 5′SS:hPrp8 cross-link (amino acids 1966–1970, underlined) were fully randomized to create two other libraries. Identical (solid), highly similar (dark-shaded), and similar (light-shaded) positions are indicated. (▾) Positions corresponding to the identified suppressor mutations. (B) Generation of mutant prp8 alleles. Wild-type PRP8 cleaved with NruI, and MscI was recombined in vivo with appropriate PCR fragments.