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Poly(A)-specific ribonuclease (PARN) is an oligomeric, processive, and cap-interacting 3' exonuclease. We have studied how the m7G(5')ppp(5')G cap structure affects the activity of PARN. It is shown that the cap has four distinct effects: (i) It stimulates the rate of deadenylation if provided in cis; (ii) it inhibits deadenylation if provided at high concentration in trans; (iii) it stimulates deadenylation if provided at low concentration in trans; and (iv) it increases the processivity of PARN when provided in cis. It is shown that the catalytic and cap binding sites on PARN are separate. The important roles of the 7-methyl group and the inverted guanosine residue of the cap are demonstrated. An active deadenylation complex, consisting of the poly(A)-tailed RNA substrate and PARN, has been identified. Complex formation does not require a cap structure on the RNA substrate. The multiple effects of cap are all accounted for by a simple, kinetic model that takes the processivity of PARN into account.