Studies with tRNA adenylyl(cytidylyl)transferase from Escherichia coli B : II. Regulation of AMP and CMP incorporation into tRNApCpC and tRNApC

Abstract

Incorporation of CMP into tRNApCpC by tRNA adenylyl(cytidylyl)transferase purified 7000-fold from Escherichia coli occurs in the absence of ATP. This abnormal reaction is prevented by the presence of ATP or by short-term incubation and is, to a large extent, influenced by enzyme concentration. More than one CMP (multiple CMP) will be accepted by tRNApC in the absence of ATP but not in its presence. This may reflect a control mechanism at the cellular level whereby the integrity of the CCA terminus of the tRNA is maintained by the ATP:CTP concentration. AMP incorporation into tRNApC or pyrophosphorolyzed tRNA is incomplete if unlimited CMP incorporation is allowed to precede it. After removal of a portion of the CCA terminus of purified tRNA Phe ( E. coli) by pyrophosphorolysis, the amino acid acceptor activity could be restored by the transferase only if both CMP and AMP incorporation occur simultaneously. Pyrophosphorolyzed purified tRNA Val ( E. coli) was also restored to its original acceptor activity by simultaneous CMP and AMP incorporation. Both UMP and AMP are incorporated by the transferase into tRNApC.