A 5'----3' exoribonuclease of human placental nuclei: purification and substrate specificity.

An exoribonuclease that hydrolyzes single-stranded RNA by a 5'----3' mode yielding 5'-mononucleotides has been purified from human placental nuclei. Chromatographic studies of crude placental nuclear extracts suggest that the enzyme is a relatively abundant nuclear RNase. Poly(A) is degraded by a processive mechanism while rRNA is degraded in a partially non-processive manner, possibly because of its secondary structure. The enzyme has an apparent molecular weight of 113,000, derived from determinations of the Stokes radius (43 A) and sedimentation coefficient (6.3 S). Substrates with 5'-phosphomonoester end groups are 10-20 times better than 5'-dephosphorylated substrates. The locale of the enzyme in nuclei of normal human cells as well as its mode of action suggest a role in nuclear RNA processing or turnover.