A photoaffinity analogue of ATP, 8-azido-adenosine 5′-triphosphate (8-NATP), was used to probe ATP-binding sites in native transcription complexes of vesicular stomatitis virus (VSV) (New Jersey serotype). The analogue was found to be a substrate for a serine/threonine protein kinase that phosphorylated both the NS and L proteins of native complexes. The analogue failed to interact with the RNA polymerase, another ATP-utilizing activity associated with the transcription complex. Kinetic analyses of both ATP and 8-NATP utilization by the protein kinase yielded biphasic saturation curves. Photolysis of 8-NATP in the presence of VSV transcription complexes resulted in selective labelling of the L protein. The photolabelling of L was saturable and apparently biphasic. Photolabelling of the L protein was significantly reduced by competition with ATP whereas other nucleoside triphosphates (GTP, UTP and CTP) were ineffective competitors. The stoichiometry of photolabelling was 0.2 at 10 µ-8NATP and 1.3 at 100 µ-ATP. These data provide chemical evidence for a virus-encoded serine/threonine protein kinase which resides on the L protein.


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