Expression of the Rous sarcoma virus env gene from a simian virus 40 late-region replacement vector: effects of upstream initiation codons.

Expression of the Rous sarcoma virus envelope gene (env) from a simian virus 40 (SV40) late-region replacement vector is dependent on the position of env within the SV40 late-region sequences. The difference in expression levels appeared to be due to differences in the efficiency with which the env-specific transcripts were translated, because transcription levels from different constructions were similar. Deletion of the nucleotides encoding the agnoprotein initiator codon, located upstream of the env sequences in the poorly expressed construct, resulted in high levels of env expression. The agnoprotein initiator codon and overlapping open reading frame thus act as strong barriers to further ribosome scanning and prevent initiation at the env AUG codon. We conclude that AUG codons present in the late region of SV40 can reduce expression of inserted genes positioned downstream. Nevertheless, intrinsic properties of the gene may determine its ultimate level of expression.