The 3' end of the avian sarcoma leukosis virus (ASLV) gag gene encodes a 124-amino-acid protease (PR) responsible for processing the gag and pol polyprotein precursors into the mature virion structural proteins and the reverse transcriptase. Here we report the synthesis of the mature ASLV PR and a nucleocapsid (NC)-PR gag precursor fragment in Escherichia coli. E. coli extracts containing mature PR correctly cleaved a synthetic decapeptide homologous to a known ASLV cleavage site. Also, the NC-PR precursor fragment appeared to be correctly processed to produce NC and PR in the bacterial cells. This cleavage was blocked by a mutation in the putative active site of PR. These results strongly support the hypothesis that PR is involved in cleaving itself from the gag precursor
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