A Ribosomal Frameshift Signal in the Polymerase-Encoding Region of the IBV Genome
Nucleotide sequencing studies on the genomic RNA of infectious bronchitis virus (IBV) has revealed the presence, at its 5′ end of two very large open reading frames (ORFS) termed F1 and F2 (Boursnell et al., 1987). The 5′ proximal ORF, Fl has the capacity to encode a polypeptide of approximately 440K, while the distal ORF, F2, which overlaps Fl by about 40 nucleotides (ntds) in a different (-1) reading frame, can encode a 300K polypeptide. Our previous work has indicated that both these open reading frames can be expressed as a single polypeptide from genomic mRNA by ribosomal frame-shifting within the overlap region (Brierley et al., 1987, 1989). This conclusion was based on experiments which showed that sequences from the F1–F2 junction region, when introduced into a heterologous mRNA could direct efficient ribosomal frame-shifting during translation in vitro and in vivo. Thus the primary translation products of IBV genomic RNA in the infected cell consists of a 440K product, corresponding to the Fl ORF, and an additional 740K ‘read-through’ product representing fusion of the F1 and F2 ORFs (Figure 1).
KeywordsInfectious Bronchitis Virus Rous Sarcoma Virus Large Open Reading Frame Pseudoknot Structure Primary Translation Product
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