Polypeptide products of nonsense mutations. II. Minor fragments produced by nonsense mutations in the z gene of the lactose operon of Escherichia coli

Untreated extracts of z-gene nonsense mutants of Escherichia coli were found to function as α-donors in a complementation reaction with XA21. In mutants which are operator-proximal to a certain point (mutant AP2177), the α-donating material was the nonsense fragment. In the region bounded by and including mutants AP2177 and NG521, extracts of mutants contained two species of polypeptides. One was the principle auto-α donor and corresponded to the nonsense fragment. A second fragment, α_U1, with a molecular weight of 46,000, functioned as the principle α-complementing material in untreated extracts. α_U1 is a minor component (~10%) of the total z-gene protein.