Reverse transcriptase of the
avian sarcoma and leukosis retroviruses is a heterodimer composed of a 63-kDa alpha and a 95-kDa beta
polypeptide chain, both of which are encoded in the pol gene and are produced by proteolytic processing of a larger precursor. We previously constructed a bacterial expression clone of the entire pol coding region that produces a
protein 4 kDa larger than the mature viral beta subunit. By use of this clone and synthetic
oligonucleotides to introduce
stop codons, two derivatives have been constructed: one that directs synthesis of a
protein equivalent to the mature beta subunit and the other that directs synthesis of a
protein equivalent to alpha subunit. Predicted amino acid sequences of these
proteins differ from their viral counterparts only by an initiator
methionine that was added to the N termini for expression in Escherichia coli. Both bacterially expressed
proteins exhibit
reverse transcriptase activity and appear to function as homodimers. The properties of these
proteins resemble those of the viral
reverse transcriptase heterodimer; however, the bacterially produced alpha dimer
protein could be distinguished from the other
proteins by its increased sensitivity to heat inactivation, which also has been reported for the corresponding viral product. These results show that correct folding and expression of enzymatic function does not require formation of a precursor. The alpha and beta clones provide a convenient source of individual
pol gene products for further evaluation of their roles in the synthesis and integration of retroviral
DNA.