Standard preparations of Escherichia coli RNA polymerase harbor a 70 kDa protein with NTPase (beta-gamma cleavage) activity that is not a recognized polymerase subunit. The NTPase activity of this component, before and after separation from the polymerase, is strongly dependent on the presence of DNA; single-stranded polydeoxynucleotides are more effective than double-stranded. ATP and GTP are cleaved, the latter much less readily. The NTPase as it occurs with the polymerase displays cleavage preference for NTPs that are not complementary to the DNA, a fact that has led to proposals for involvement of the NTPase in transcriptional error prevention [Volloch, V. Z., Rits, L. & Tumerman, L. (1979) Nucleic Acids Res. 6, 1535-1546; Libby, R. T., Nelson, J. L., Calvo, J. M., & Gallant, J. A. (1989) EMBO J. 8, 3253-3158]. We find, however, that the lesser cleavage in the presence of complementary DNA results from competition for the NTP between the processes of incorporation by the polymerase and of cleavage by the NTPase, operating on the same substrate pool. The greater cleavage with noncomplementary DNA occurs because of the lack of incorporation by the polymerase, which then does not compete with the NTPase for the substrate pool. Thus, these findings indicate that the cleavage preference of the NTPase for noncomplementary NTPs is not part of a mechanism for error prevention during transcription.