TRANSLATION OF THE REOVIRUS M1 GENE INITIATES FROM THE FIRST AUG CODON IN BOTH INFECTED AND TRANSFECTED CELLS

Reovirus mu 2 protein can be expressed via the mouse phosphoglycerate kinase promoter to low levels in stably transfected L cells. To increa se mu 2 expression, the terminal regions of the M1 gene cDNA construct s were modified and the effect on mu 2 expression was analyzed. The M1 gene has a single large open reading frame beginning at nucleotide 14 with another, in frame, AUG codon at nucleotide 161 reported to be us ed for translation initiation. Unexpectedly, deletions of the Evil 5' terminal sequence upstream of the reported translation initiation codo n, AUG(161), resulted in loss of detection of mu 2 expression. When ex pression was driven by the stronger T7 promoter in the presence of rec ombinant vaccinia virus expressing the T7 RNA polymerase, constructs w ith the M1 5'-terminal deletion produced a smaller protein product of similar to 68 kDa, compared to similar to 73 kDa for the protein produ ced from the full-length M1-containing constructs consistent with the loss of 49 amino acids. The amount of shorter mu 2 product was increas ed by producing an improved 'Kozak' consensus sequence around the AUG codon at nucleotide 161 or by introducing an internal ribosome entry s ite at this location. Full-length M1 gene constructs produced a protei n of the same size as the authentic mu 2 protein from virus-infected c ells. It was further shown that the similar to 73 kDa product was expr essed when the M1 gene was in different plasmid backgrounds and even w hen the M1 gene transcript was preceded by a 1 kb gene. This study dem onstrated that translation of the reovirus M1 gene initiates from the first AUG codon in both infected and transfected cells.