ACCELERATED MESSENGER-RNA DECAY IN CONDITIONAL MUTANTS OF YEAST MESSENGER-RNA CAPPING ENZYME

Current models of mRNA decay in yeast posit that 3' deadenylation prec edes enzymatic removal of the 5' cap, which then exposes the naked end to 5' exonuclease action. Here, we analyzed gene expression in Saccha romyces cerevisiae cells bearing conditional mutations of Ceg1 (cappin g enzyme), a 52 kDa protein that transfers GMP from GTP to the 5' end of mRNA to form the GpppN cap structure. Shift of ceg1 mutants to rest rictive temperature elicited a rapid decline in the rate of protein sy nthesis, which correlated with a sharp reduction in the steady-state l evels of multiple individual mRNAs. ceg1 mutations prevented the accum ulation of SSA1 and SSA4 mRNAs that were newly synthesized at the rest rictive temperature. Uncapped poly(A)(+) SSA4 mRNA accumulated in cell s lacking the 5' exoribonuclease Xrn1. These findings provide genetic evidence for the long-held idea that the cap guanylate is critical for mRNA stability. The deadenylation-decapping-degradation pathway appea rs to be short-circuited when Ceg1 is inactivated.