ON MALLEABILITY IN THE GENETIC-CODE

To explain now-numerous cases of codon reassignment (departure from th e ''universal'' code), we suggest a pathway in which the transformed c odon is temporarily ambiguous. All the unusual tRNA activities require d have been demonstrated. In addition, the repetitive use of certain r eassignments, the phylogenetic distribution of reassignments, and the properties of present-day reassigned tRNAs are each consistent with ev olution of the code via an ambiguous translational intermediate.