Cytidines in tRNAs that are required intact by ATP/CTP:tRNA nucleotidyltransferases from Escherichia coli and Saccharomyces cerevisiae.

Individual species of tRNA from Escherichia coli were treated with hydrazine/3 M NaCl to modify cytidine residues. The chemically modified tRNAs were used as substrate for ATP/CTP: tRNA nucleotidyltransferases from E. coli and yeast, with alpha-32P]ATP as cosubstrate. tRNAs that were labeled were analyzed for their content of modified cytidines. Cytidines at positions 74 and 75 were found to be required chemically intact for interaction with both enzymes. C56 was also required intact by the E. coli enzyme in all tRNAs, and by the yeast enzyme in several instances. C61 was found to be important in seven of 14 tRNAs with the E. coli enzyme but only in four of 13 tRNAs with that from yeast. Our results support a model in which nucleotidyltransferase extends from the 3' end of its tRNA substrate across the top of the stacked array of bases in the accepter- and psi-stems to the corner of the molecule where the D- and psi-loops are juxtaposed.