A photoactive nucleotide analogue of UTP, 5-azidouridine 5′-triphosphate (5-NUTP), has been demonstrated to interact with the RNA polymerase of the vesicular stomatitis virus (VSV) transcription complex. Kinetic studies indicated that 5-NUTP served as an efficient replacement for UTP in polymerase reactions. The for the azido analogue was 27 µ and that of the natural substrate, UTP, was 7 µ. Photolysis of [γ-P]5-NUTP in the presence of VSV transcription complexes resulted in selective radiolabelling of the L protein. This photolabelling was saturable with an apparent of 28 µ. The L protein was protected from [γ-P]5-NUTP-mediated photolabelling by competing natural substrates (UTP, CTP, ATP, GTP). The stoichiometry of photoprobe incorporation into the transcription complex was close to unity with respect to the L protein. These data provide evidence that the nucleotide-binding domain of the VSV RNA polymerase contains amino acid residues of the L protein.


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