SUMMARY: Serine acetyltransferase (SAT) from is subject to feedback inhibition by -cysteine. A mutant was isolated which excretes -cysteine because of a lesion in , the structural gene for SAT, rendering the enzyme less feedback sensitive. To analyse the structural basis for this mutation the genes both from wild-type and the mutant strain were cloned and their nucleotide sequences determined. The gene contained an open reading frame consisting of 819 bp, equivalent to a protein of 273 amino acids. The mutant gene showed a single base change in position 767 resulting in a methionine to isoleucine substitution. A causal connection between this SAT sequence alteration, feedback insensitivity and -cysteine excretion was demonstrated. The SAT from the wild-type strain was purified. It was composed of a single polypeptide chain migrating in SDS gels according to an of 34000. As in , the enzyme was associated in a bifunctional complex with -acetylserine (thiol)-lyase.


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