Suppression of multiple substrate mutations by spliceosomal prp8 alleles suggests functional correlations with ribosomal ambiguity mutants

Conformational change within the spliceosome is required between the first catalytic step of pre-mRNA splicing, when the branch site (BS) attacks the 5' splice site, and the second step, when the 5' exon attacks the 3' splice site, yielding mRNA and lariat-intron products. A genetic screen for suppressors of BS A-to-G mutants, which stall between the two steps, identified Prp8, the highly conserved spliceosomal factor. prp8 suppressors facilitate the second step for multiple intron mutants and interact functionally with first step suppressors, alleles of PRP16 and U6 snRNA. Genetic interactions among prp8, prp16, and U6 alleles suggest that these factors control a common stage in first-to-second step transition. We propose that mutant substrates are utilized by alteration of the equilibrium between first/second step conformations, resembling tRNA miscoding caused by altered equilibrium between open/closed ribosomal conformations. This mechanistic commonality suggests that alteration of rearrangements represents an evolutionarily convenient way of modulating substrate selectivity.