AMINO ACID REPLACEMENTS OF GLUTAMIC ACID RESIDUE AT POSITION 48 IN TRYPTOPHAN SYNTHETASE A PTEIN OF ESCHERICHIA COLI

Five independently isolated mutants have alterations of the glutamic acid codon specifying position 48 of the wild-type tryptophan synthetase A protein (Escherichia coli). Mutants A3 and A33 have replacements of the glutamic acid residue by valine and methionine, respectively, while A1 and A88 are both amber mutants. In mutant A11, the same glutamic acid residue is probably replaced by a glutamine residue. The introduction of the trp-linked amber suppressor from strain Ymel into mutant A88 leads to the insertion of tyrosine at position 48 in the A protein. The antipolar effect resulting from the presence of the A88 alteration is markedly reduced by the presence of the amber suppressor. The probable codon changes characteristic of mutants A1, A3, A11 and A88 are consistent with mutational events involving single base-pair changes. The replacement of glutamic acid by methionine in mutant A33 can only be accounted for by a change of two, adjacent, base pairs. Mutant A33 is exceptional in that it does not revert to prototrophy. Marked differences in recombination frequencies are observed when these mutants, altered in the same codon, are crossed with other A gene mutants. © 1968.