MESSENGER-RNA DEADENYLYLATION PRECEDES DECAPPING IN MAMMALIAN-CELLS

In yeast, the major mRNA degradation pathway is initiated by poly(A) t ail shortening that triggers mRNA decapping, The mRNA is then degraded by 5'-to-3' exonucleolysis, In mammalian cells, even though poly(A) t ail shortening also precedes mRNA degradation, the degradation pathway has not been elucidated, We have used a reverse transcription-PCR app roach that relies on mRNA circularization to measure the poly(A) tail length of four mammalian mRNAs, This approach allows for the simultane ous analysis of the 5' and 3' ends of the same mRNA molecule, For all four mRNAs analyzed, this strategy permitted us to demonstrate the exi stence of small amounts of decapped mRNA species which have a shorter poly(A) tail than their capped counterparts, Kinetic analysis of one o f these mRNAs indicates that the decapped species with a short poly(A) tail are mRNA degradation products, Therefore, our results indicate t hat decapping is preceded by a shortening of the poly(A) tail in mamma lian cells, as it is in yeast, suggesting that this mRNA degradation p athway is conserved throughout eukaryotic evolution.