Complementary synthetic oligonucleotides (63
bases) encoding the first 9 amino acids of MHV open reading frame 4a (ORF4a)
(including the initial AUG codon) followed by the 9 amino acids of gp33 were
synthesized such that, when hybridized, the 5 and 3 ends of the double-stranded
fragment contained sequences complementary to SalI- and BamHI-cleaved
DNA ends, respectively