Decapping of stabilized, polyadenylated mRNA in yeast pab1 mutants

Interaction of the poly(A) binding protein, Pab1p, with mRNA plays an important role in gene expression. This work describes an analysis of pab1 mutants in Saccharomyces cerevisiae. Yeast pab1 mutants were found to be sensitive to elevated concentrations of copper (Cu) and 3-aminotriazole (3-AT) in the growth medium. This phenotype arises because these pab1 mutants underaccumulate mRNA, including the CUP1 and HIS3 mRNAs, the products of which are required for Cu and 3-AT resistance, respectively. To determine the cause of the mRNA underaccumulation, mRNA turnover and production were examined in the pab1-53 mutant. It was found that although the pattern of mRNA decay was altered, and substantial decapping of polyadenylated mRNA could be detected, mRNA was not destabilized in this strain. It was also found that the pab1 mutant was impaired in the production of mRNA. These data show that the decreased level of mRNA in the pab1-53 mutant arises from poor production, and they suggest that yeast Pab1p is involved in an aspect of nuclear mRNA metabolism. They also indicate that deadenylation can be uncoupled from decapping without significant changes in an mRNA's stability, and that this uncoupling can be tolerated by yeast.