The ability to express the genes of pathogenic human viruses, such as the acquired immune deficiency syndrome (AIDS) virus (also called human immunodeficiency virus) in bacterial cells affords the opportunity to study proteins that are ordinarily difficult or inconvenient to obtain in amounts sufficient for detailed analysis. A segment of the AIDS virus pol gene was expressed in Escherichia coli. Expression resulted in the appearance of reverse transcriptase activity in the bacterial cell extracts. The extracts contained two virus-related polypeptides that have the same apparent molecular weights as the two processed forms of virion-derived reverse transcriptase (p66 and p51). The formation of these two polypeptides depended on the coexpression of sequences located near the 5' end of the pol gene, a region that is thought to encode a viral protease. This bacterial system appears to generate mature forms of the AIDS virus reverse transcriptase by a proteolytic pathway equivalent to that which occurs during virus infection of human cells.

Processing of the retroviral gag and pol gene products is mediated by a viral protease. Bacterial expression systems have been developed which permit genetic analysis of the human immunodeficiency virus type 1 protease as measured by cleavage of the pol protein precursor. Deletion analysis of the pol reading frame locates the sequences required to encode a protein with appropriate proteolytic activity near the left end of the pol reading frame but largely outside the gag-pol overlap region, which is at the extreme left end of pol. Most missense mutations within an 11-amino-acid domain highly conserved among retroviral proteases and with sequence similarity to the active site of aspartic proteinases abolish appropriate processing, suggesting that the retrovirus proteases share a catalytic mechanism with aspartic proteinases. Substitution of the amino acids flanking the scissile bond at three of the processing sites encoded by pol demonstrates distinct sequence requirements for cleavage at these different sites. The inclusion of a charged amino acid at the processing site blocks cleavage. A subset of these substitutions also inhibits processing at the nonmutated sites.