An immunological study of mutants of Escherichia coli lacking the enzyme tryptophan synthetase.

Previous studies with mutants of Neurospora crassa lacking the enzyme tryptophan synthetase (indole + L-serine -> L-tryptophan) have shown that certain of these strains form large amounts of a protein which is immunologically cross reacting and hence probably structurally related to the enzyme (Suskind et al., 1955). One mutant strain which, by genetic criteria, was allelic with' the other mutants (Newmeyer, 1954; Yanofsky and Bonner, 1955) was found to be incapable of forming either tryptophan synthetase or the immunologically related protein (Suskind et al., 1955). On the basis of these observations it was tentatively concluded that the cross reacting material probably represented an inactive or incomplete form of the enzyme (Suskind et al., 1955; Yanofsky, 1956; Suskind, 1957a). Other interpretations are also possible, however; the cross reacting material could be an adaptive enzyme induced by the indole accumulated by these strains, it could be one component of a tryptophan synthetase "complex" if the tryptophan synthetase reaction actually required two enzymes, or it could be a protein or enzyme structurally similar to tryptophan synthetase but controlled by a gene distinct from the gene controlling tryptophan synthetase formation. The present study was initiated in an attempt to distinguish among these various possibilities. Escherichia coli was selected as the experimental organism because the enzymatic steps involved in tryptophan synthesis are better understood in E. coli than in N. crassa, and because it was of interest to determine whether tryptophan synthetase lacking mutants of an unrelated organism would also form a protein immunologically related to this enzyme.