Mechanism of transcription of Paramyxovirus genome : Purification and characterization of host factors

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Sendai virus (SeV), a member of Paramyxovirus family, contains a monopartite negative strand RNA genome of approximately 15 kilobases. To elucidate the mechanism of transcription and replication of SeV, we developed an efficient and faithful in vitro transcription system using purified virus particles. We have previously shown that in vitro mRNA synthesis of SeV is almost entirely dependent on the addition of cellular proteins (host factors). In the present study, we further furified and characterized the host factor activity, and obtained the following results. 1) We purified the host factor activity, which stimulates in vitro SeV mRNA synthesis, from bovine brain, and found that the activity consists of four complementary protein factors, tubulin, phosphoglycerate kinase, enolase, and unidentified p24. 2) We showed that tubulin is involved in a transcription initiation complex formation by removing the viral M protein from the RNP, and by being integrated itself into the complex. On the other hand, phosphoglycerate kinase, enolase, and p24 are found to act at RNA shain elongation step. Furthermore, 3) we demonstrated that a recombinant viral L protein exhibits the cap methylase (mRNA guanine-7-methyltransferase) activity.