The roles of three enzymes in messenger RNA degradation in cell-free systems from normal or phage-infected Escherichia coli

A microsomal exoribonuclease from rat liver

A exoribonuclease has been purified from the microsomes of rat liver. The enzyme had an apparent molecular weight of 80 000-83 000 and produced, via a processive mechanism, 5'-AMP as the only product from poly(A). The degradation was found to proceed in the 3' to 5' direction. The relative rates of breakdown of synthetic polynucleotides by the enzyme under standard conditions were in the order poly(A) equal to poly(U) greater than poly(C). In addition to Mg2+, K+ was required for maximum activity. The enzymic activity was inhibited by p-chloromercuribenzoate and poly(G), but not by a rate liver RNAase inhibitor. The effect of spermine on the breakdown of synthetic polynucleotides by the enzyme has been studied. In the absence of K+, the breakdown of poly(C) was stimulated and that of poly(A) was stimulated slightly. However, the breakdown of poly(U) was inhibited slightly by spermine.

A study of the localization of polynucleotide phosphorylase within rat liver cells and of its distribution among rat tissues and diverse animal species

1. Rat liver polynucleotide phosphorylase was localized in the mitochondrion, but may also occur in the nucleus. 2. The mitochondrial enzyme was found in rat heart, kidney, liver, muscle and spleen. 3. Mitochondrial polynucleotide phosphorylase is also present in calf, chicken, guinea-pig and rabbit liver and in goldfish muscle. 4. A possible physiological role for the enzyme in the control of the intramitochondrial ADP concentration is suggested.