The RABV L protein generates the mRNA cap structure by an unconventional mechanism: (a) The recombinant RABV L protein (0.1 μg) was incubated with the indicated substrates under the standard conditions for the VSV L protein. Nuclease P1-digests of RNA products were analyzed by thin layer chromatography on a polyethyleneimine-cellulose plate (PEI-cellulose TLC), followed by autoradiography. The positions of the origin (ori.), guanosine nucleotides (GMP, GDP, GTP), and cap structures (GpppA, GppppA) are shown; (b) The recombinant RABV L protein (lane 2, 0.1 μg; lane 3, 0.2 μg) was incubated with [γ-32P]GTP. The reaction mixtures were analyzed by PEI-cellulose TLC followed by autoradiography. Lane 1 indicates no enzyme. Lane M shows the position of 32P-labeled inorganic phosphate (Pi), which was generated by digestion of [γ-32P]GTP with alkaline phosphatase.