title: Lack of correlation between Sendai virus P/C mRNA structure and its utilization of two AUG start sites from alternate reading frames: implications for viral bicistronic mRNAs.
abstract: The polycistronic P/C mRNA of Sendai virus encodes five proteins (C', P, C, Y1, and Y2) each of which initiates from a distinct start site. Two major proteins, P and C, are expressed in approximately equimolar amounts from two consecutive AUGs in overlapping reading frames. To better understand the mechanism of expression of the C protein from a downstream AUG, site-directed mutants of the P/C mRNA were created and expressed in COS1 cells. The secondary structure of the mRNA was examined to determine whether the mRNA structure played any role in the synthesis of the C protein. Our results ruled out any significant involvement of the 5' UTR, sequence contexts, secondary structure, distance between the start sites, and sequences downstream to the C-AUG. However, they are consistent with the concept that the synthesis of the C protein is primarily dependent on the orientation of its reading frame, i.e., +1 in relation to the upstream P reading frame. The downstream reading frame was translated poorly when it occurred in +2 orientation in relation to the upstream reading frame. Interestingly, all the known functional bicistronic mRNAs with overlapping reading frames from cytoplasmic RNA viruses have their downstream reading frame in +1 orientation relative to the upstream frame. We propose that the evolutionary conservation of the downstream reading frame in +1 orientation in these bicistronic mRNAs is important for its efficient translation.
authors.affiliation: Department of Immunology/Microbiology, Rush Medical College, Chicago, Illinois 60612, USA.
authors.forename: K C
authors.initials: KC
authors.lastname: Gupta
authors.forename: E
authors.initials: E
authors.lastname: Ono
authors.forename: X
authors.initials: X
authors.lastname: Xu
mesh_terms: D001483:Base Sequence; D018387:Codon, Initiator; D004252:DNA Mutational Analysis; D005814:Genes, Viral; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009690:Nucleic Acid Conformation; D010222:Parainfluenza Virus 1, Human / Q000235:genetics*; D010442:Peptide Chain Initiation, Translational; D010750:Phosphoproteins / Q000096:biosynthesis / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics*; D012367:RNA, Viral / Q000235:genetics*; D016364:Reading Frames / Q000235:genetics*; D014158:Transcription, Genetic; D014764:Viral Proteins / Q000096:biosynthesis / Q000235:genetics
chemical_list: D018387:Codon, Initiator; C055859:P protein, Sendai virus; D010750:Phosphoproteins; D012333:RNA, Messenger; D012367:RNA, Viral; D014764:Viral Proteins; C051221:nonstructural C protein, Sendai virus