A self-aminoacylating RNA catalyst is shown to carry out the chemistry required for turnover, being reacylated several times from aminoacyl-AMP with an unaltered rate, thereby meeting one definition of an enzyme. Furthermore, a newly applied gel electrophoresis assay suggests first order kinetics in RNA and saturation kinetics in the substrate aminoacyl-adenylate, implying a Michaelis complex. AMP is a competitive inhibitor, though phenylalanine is not detectably inhibitory, consistent with a Michaelis complex through the AMP moiety of phenylalanyl-adenylate substrate. This idea is supported by measurement of elevated acylation velocities with seryl and alanyl-adenylates. The rate of aminoacylation increases with pH, consistent with attack of a terminal ribose oxyanion on the carbonyl carbon atom of the adenylate.