3′-Exonucleolytic removal of the poly (A) tail is the first and often rate-limiting step in the
decay of many eucaryotic mRNAs. In a cytoplasmic extract from HeLa cells, the poly (A) tail
of mRNA was degraded from the 3′-end. In agreement with earlierin vivo observations,
prominent decay intermediates differed in length by about 30 nucleotides. The Mg 2+-
dependent, poly (A)-specific 3′-exoribonuclease responsible for this poly (A) shortening
activity was purified from calf thymus. A polypeptide of 74 kDa copurified with the activity …