TRANSLATION INITIATION AT ALTERNATE IN-FRAME AUG CODONS IN THE RABIESVIRUS PHOSPHOPROTEIN MESSENGER-RNA IS MEDIATED BY A RIBOSOMAL LEAKY SCANNING MECHANISM

The phosphoprotein of rabies virus is a 297 amino-acid polypeptide enc oded by the longest open reading frame of the P gene. Immunoprecipitat ion experiments using a monoclonal antiserum directed against the P pr otein detected the P protein and at least four additional shorter prod ucts in infected cells, cells transfected with a plasmid encoding the wild-type P protein, and purified virus (CVS strain). By means of dele tion analyses, these proteins were shown to be translated from seconda ry downstream in-frame AUG initiation codons. Immunofluorescence exper iments indicated that all these P products were found in the cytoplasm of transfected cells; however, the proteins initiated from the third, fourth, and fifth AUG codons were found mostly in the nucleus. Change s in the 5'-terminal region of the P mRNA (including site-specific mut ations, deletions, and insertions) demonstrated that a leaky scanning mechanism is responsible for translation initiation of the P gene at s everal sites.