An
RNA-dependent RNA polymerase is packaged within the virions of purified
vesicular stomatitis virus, a nonsegmented negative-strand RNA virus, which carries out transcription of the genome
RNA into mRNAs both in vitro and in vivo. The
RNA polymerase is composed of two virally encoded
polypeptides: a large
protein L (240 kDa) and a
phosphoprotein P (29 kDa). Recently, we obtained biologically active L
protein from insect cells following
infection by a recombinant baculovirus expressing L gene. During purification of the L
protein from Sf21 cells, we obtained in addition to an active L fraction an inactive fraction that required uninfected insect
cell extract to restore its activity. The cellular factors have now been purified, characterized, and shown to be beta and gamma subunits of the
protein synthesis
elongation factor EF-1. We also demonstrate that the alpha subunit of
EF-1 remains tightly bound to the L
protein in the inactive fraction and betagamma subunits associate with the L(alpha) complex. Further purification of L(alpha) from the inactive fraction revealed that the complex is partially active and is significantly stimulated by the addition of betagamma subunits purified from Sf21 cells. A putative inhibitor(s) appears to co-elute in the inactive fraction that blocked the L(alpha) activity. The purified virions also package all three subunits of
EF-1. These findings have a striking similarity with
Qbeta RNA phage, which also associates with the bacterial homologue of
EF-1 for its replicase function, implicating a possible evolutionary relationship between these host
proteins and the
RNA-dependent RNA polymerase of RNA viruses.