Mutation of a termination codon affects src initiation.

Abstract

The four Rous sarcoma virus messages gag, gag-pol, env, and src all derive from a fuli-length RNA precursor. All four messages contain the same 5' leader segment. Three of the messages, gag, gag-pol, and env, use an AUG present in this leader to initiate translation. The src AUG initiation codon lies 3' of the leader segment, 90 bases downstream of the gag initiation codon in the spliced src message. However, in the spliced src message a UGA termination codon lies between the gag AUG and the src AUG. All three codons are in the same reading frame. By using oligonucleotide-directed mutagenesis, the UGA termination codon has been converted to CGA. Cells infected with the mutant (called 1057 CGA) were spindle shaped, distinct from the rounded shape of cells infected with the parental Rous sarcoma virus. The mutant virus initiates src ranslation at the gag AUG, producing a 63,000-dalton src protein. We suggest that the wild-type src message produces two polypeptides, a very small (nine-amino acid) peptide that is initiated at the gag AUG and the 60,000-dalton src protein that is initiated at the src AUG.Rous sarcoma virus (RSV) is apparently transcribed into a single RNA species beginning at the left end of the R sequence in the left-hand long terminal repeat and terminating at the right end of the R sequence in the right-hand long terminal repeat. This transcript constitutes genomic RNA and serves as the precursor for RSV messenger RNAs. Four messenger RNAs are known: gag mRNA, which is apparently unspliced and is thought to be identical to genomic RNA; gag-pol mRNA, which may have a small region near the gag-pol junction removed by RNA splicing; env mRNA, which has most of gag and pol removed by splicing; and src mRNA, which has most of gag, pol, and env removed by splicing (30; Fig.

Role of the avian retrovirus mRNA leader in expression: evidence for novel translational control.

“…The fact that several mRNAs (including ASV) contain optimal AUGs upstream of the major reading frame initially seemed paradoxical. It now is clear that in some cases upstream AUGs can actually be recognized as initiator codons and that downstream reading frames may then be recognized through reinitiation (10,11,18,21,27). Furthermore, upstream AUGs, whether followed by in-frame termination codons (11) or not (18,21), can act as barriers of downstream initiation; the most efficient barriers are those which contain an AUG in the optimal initiation context.…”

“…In two exceptional cases, poliovirus and avian sarcoma virus (ASV) mRNAs, AUGs preceded by consensus initiator nucleotides are found within what are predicted to be the 5' noncoding regions of the mRNAs. It is possible that upstream AUGs may be recognized as start codons, initiating the translation of a short open reading frame, as suggested in the case of ASV mRNAs (10,27 the riRNA, as has been suggested in the cases of both poliovirus (25) and ASV (5).…”

Sendai virus C proteins are categorically nonessential gene products but silencing their expression severely impairs viral replication and pathogenesis

“…When an AUG codon upstream from the start of the major protein coding sequence lies in a favourable context, thereby precluding leaky scanning, ribosomes can still reach the downstream initiation site, provided that a termination codon occurs in-frame with the first AUG codon, and upstream from the second (Hughes et al 1984: Kozak 1984Liu et al 1984). In such cases the first ORF is invariably small, encoding only a small peptide, and translated, after which 40S ribosomal subunits apparently resume scanning and reinitiate further downstream (reviewed in Kozak 1989).…”

Functional expression of the leftward open reading frames of the A component of tomato golden mosaic virus in transgenic tobacco plants.

“…AL1, by virtue of being the first open reading frame, and AL3, which may be optimized for internal translation initiation, may both be translated efficiently from the polycistronic RNA, whereas AL2 may be translated poorly. The AL1 and AL2 open reading frames overlap, and upstream AL1 translation may interfere with the initiation of AL2 translation (Hughes et al, 1984;Liu, Simonsen, and Levinson, 1984;Peabody, Subramani, and Berg, 1986), further reducing the efficiency of AL2 expression in the transgenic plants. The AL1 and AL3 open reading frames do not overlap, and ALI translation would not interfere with the initiation of AL3 translation.…”