The tRNA 3' end contains the conserved CCA sequence at the 74-76 positions.
The CCA sequence is synthesized and maintained by the CCA-adding enzymes.
The specificity of the Escherichia coli enzyme at each of the 74-76 positio
ns was investigated using synthetic minihelix substrates that contain permu
ted 3' ends. Results here indicate that the enzyme has the ability to synth
esize unusual 3' ends. When incubated with CTP alone, the enzyme catalyzed
the addition of C74, C75, C76, and multiple Cs. Although the addition of C7
4 and C75 was as expected, that of C76 and multiple Cs was not. In particul
ar, the addition of C76 generated CCC, which would have conflicted with the
biological role of the enzyme. However, the presence of ATP prevented the
synthesis of CCC and completely switched the specificity to CCA. The presen
ce of ATP also had an inhibitory effect on the synthesis of multiple Cs. Th
us, the E. coli CCA enzyme can be a poly(C) polymerase but its synthesis of
poly(C) is regulated by the presence of ATP. These features led to a model
of CCA synthesis that is independent of a nucleic acid template. The synth
esis of poly(C) by the CCA-adding enzyme is reminiscent of that of poly(A)
by poly(A) polymerase and it provides a functional rationale for the close
sequence relationship between these two enzymes in the family of nucleotidy
ltransferases.