The sequence of a 5′-proximal region of mRNA 5 of coronavirus MHV-JHM was determined by chain-terminator sequencing of cDNA subcloned in M13. The sequence contained two long open reading frames of 321 bases and 264 bases, overlapping by five bases but in different frames. Both open reading frames are initiated by AUG codons in sequence contexts that are relatively infrequently used as initiator codons. The smaller, downstream open reading frame encoded a neutral protein (mol. wt. 10200) with a hydrophobic amino terminus. The larger, 5′-proximal open reading frame encoded a basic protein (mol. wt. 12400) which lacks internal methionine residues. With the exception of the AUG codon initiating the downstream open reading frame, no internal AUG codons were found within the sequence covered by the upstream open reading frame. These results suggest that the MHV-JHM mRNA 5 is translated to produce two proteins by a mechanism involving internal initiation of protein synthesis. Preliminary evidence is presented showing that the downstream open reading frame is functional .


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