Molecular heterogeneity in the amino-terminal region of alkaline phosphatase

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Abstract

It was reported earlier that a phosphatase nonsense mutant produces more than one molecular species of phosphatase fragment, a surprising result since the Escherichia coli chromosome contains only one phosphatase structural gene. The present experiments show that different species of fragments from the same mutant have different amino-terminal sequences. In the ochre mutant S10, two of the fragments differ slightly in molecular size; two residues present in the larger fragment, namely the amino-terminal histidine and an internal (probably adjacent) valine, are missing from the amino-terminal region of the smaller fragment. As an explanation for this observation, we postulate that the nonsense mutant initially produces one uniform species of fragment, and that different species are generated subsequently by the action of an aminopeptidase which releases a variable number of residues from the fragment molecules. This mechanism could also account for the occurrence, in phosphatase-positive strains, of different species of isozymes of the standard phosphatase molecule.

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This research was supported by grants from the National Science Foundation and the National Institutes of Health, U.S. Public Health Service.

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