The effect of MPA on the capping of mRNAs was then assessed in vivo. A primer extension assay was used to monitor the effect of MPA on the formation of the RNA cap structure in S. cerevisiae. Previous studies have shown that during cDNA synthesis by reverse transcriptase, the presence of a cap structure results in the synthesis of products that harbor an extra 39 nucleotide [40, 59] . We therefore performed primer extension assays with a 59 P 32labeled 18-mer oligonucleotide complementary to the 59 region of the SSA1 mRNA. The oligonucleotide was annealed to total mRNAs extracted from cells that were grown in the presence or absence of 500 mg/ml MPA for 3 h, and extended with reverse transcriptase. Our results indicate that cDNA products with apparent chain lengths of 71 and 72 nucleotides, corresponding to uncapped and capped mRNAs, were synthesized using mRNAs extracted from untreated cells (Fig. 8) . However, treatment with MPA resulted in a marked reduction of the 72 nucleotides species, indicating that cap formation was inhibited in the presence of MPA. Quantitative analysis indicate that cap formation was reduced by 50% in the presence of 500 mg/ml MPA (Fig. 8) . We conclude that the addition of MPA to S. cerevisiae cells leads to an inhibiton of RNA cap synthesis.