Final answer:
Exonucleases are the enzymes responsible for shortening the polyA tail by cleaving nucleotides from the end, thus removing the PABP. They play a role in the mRNA decay process, which is crucial for gene expression regulation.
Step-by-step explanation:
The enzymes that shorten the polyA tail by chopping it up and removing the Poly(A) Binding Protein (PABP), which no longer has substrate to bind to, are exonucleases. Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end of a polynucleotide chain. They are distinct from endonucleases, which cleave the phosphodiester bonds within a DNA or RNA strand; ligases, which join two strands of nucleic acids together, and kinases, which transfer a phosphate group from a high-energy molecule, like ATP, to a substrate molecule.
Specifically, in the context of RNA processing, the enzymes responsible for the degradation of the polyA tail, leading to the removal of PABP, are 3' to 5' exonucleases. The removal of the tail is part of the process of mRNA decay, which is an important regulatory mechanism in the control of gene expression.