CONDITIONAL MUTANTS OF THE YEAST MESSENGER-RNA CAPPING ENZYME SHOW THAT THE CAP ENHANCES, BUT IS NOT REQUIRED FOR, MESSENGER-RNA SPLICING

The 5' end of eukaryotic mRNAs are modified by the addition of a 7-met hyl guanosine (m(7)G) cap. The role of the cap in translation has been well established. Additionally, studies conducted in vitro or in micr oinjected Xenopus oocytes have implicated the cap in RNA Processing an d transport. To determine the fate of uncapped mRNA in intact yeast ce lls, conditional alleles of the gene encoding the capping enzyme guany lyltransferase subunit (CEG1) were generated. RNA analysis of temperat ure-sensitive ceg1 strains revealed an accumulation of unspliced pre-m RNAs and a corresponding decrease in spliced mRNAs at the restrictive temperature. A substantial proportion of spliced mRNA was also uncappe d, Therefore, the cap appears to stimulate, but is not absolutely requ ired for, splicing in vivo. In addition, steady-state levels of severa l mRNAs were decreased, perhaps due to increased degradation of uncapp ed mRNAs. In contrast to splicing, mRNA polyadenylation and transport to the cytoplasm were unaffected.