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Ribosome-bound Polyriboadenylate Polymerase

From the ¹ From the Department of Molecular Biology, Albert Einstein College of Medicine, New York, New York 10461

The relationship in vitro of the enzyme polyriboadenylate polymerase to ribonucleoprotein particles was investigated. The enzyme was predominantly bound to 70 S ribosomes in solutions containing 10 mM MgCl₂, and to 50 S ribosomes in 1 mM and 0.1 mM MgCl₂. Little if any enzymic activity was found associated with 100 S ribosomes. Dissociation of the enzyme from ribosomes in low concentrations of MgCl₂ was reversible. Purified polyriboadenylate polymerase was also found to bind to 70 S, 50 S, and 30 S particles.

The polyriboadenylate product of the reaction catalyzed by the ribosomal enzyme was bound to ribonucleoprotein particles in a rapidly sedimenting complex. With increasing time of incubation the sedimentation rate of this complex increased. After 5 min, it was about 130 S; 20 min, 190 S; and 60 min, greater than 300 S. This increase in sedimentation velocity of the polyriboadenylate-ribosome complex appeared to be due to aggregation of polyriboadenylate rather than to chain prolongation; the average polymer size between 5 to 60 min of incubation remained constant at approximately 200 adenylate units. The sedimentation coefficient of polyriboadenylate dissociated from ribosomes in 0.1 mM MgCl₂ was 5 to 8 S. The rapid sedimentation of the complex was not changed by pancreatic ribonuclease, but after limited incubation with micrococcal nuclease, polyriboadenylate sedimented with 70 to 100 S ribosomes. Although less than 1% of total ribosomes was present in the polyriboadenylate-ribosome complex, these ribosomes were associated with most of the enzymic activity.

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