1. Radioactivity was incorporated into 5′-O-phosphoryladenylyl-(3′-5′)-adenosine (pApA) on incubation with adenylate kinase and [γ−32P]ATP. The corresponding triadenylate and tetra-adenylate reacted more slowly. 2. Only oligoadenylate with a terminal 5′-phosphate served as the substrate. 3. The product formed from pApA was shown to behave like 5′-O-pyrophosphoryladenylyl-(3′-5′)-adenosine (ppApA) on hydrolysis with alkaline phosphatase, potassium hydroxide and hydrochloric acid. 4. The characteristics of the reaction indicated that it was catalysed by adenylate kinase, but the rate of phosphate transfer from ATP to pApA was about 0·01% of that in the typical reaction with AMP. 5. The reverse reaction between ADP and ppApA occurred readily, but no additional phosphorylation of ppApA (to give pppApA) could be demonstrated.

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