Influences of Various Amino Acids on Tryptophan-Mediated Control of the Tryptophan Biosynthetic Enzymes in Escherichia coli

AUTOR(ES)

Stubbs, John D.

RESUMO

Lysates of Escherichia coli Ymel obtained from cultures grown in the absence of tryptophan in minimal medium supplemented with 0.1% casein hydrolysate show an approximate fivefold increase in steady-state specific activity of both anthranilate synthetase and tryptophan synthetase A protein relative to cultures grown in nonsupplemented medium. In the presence of repressing levels of exogenous tryptophan, growth of cultures in casein hydrolysate-supplemented medium results in a noncoordinate enhancement of repression of 10-fold for anthranilate synthetase and twofold for tryptophan synthetase A protein. Similar, but less pronounced, effects are shown for strain W3110. Strains possessing tryptophan regulator gene mutations do not exhibit this first effect, but do yield an approximate twofold decrease in specific activity of both enzymes when grown in medium supplemented with tryptophan and casein hydrolysate. A stimulation of derepression of both enzymes in strain Ymel equivalent to that induced by casein hydrolysate can be reproduced by growth in minimal medium supplemented with threonine, phenylalanine, tyrosine, serine, glutamic acid, and glutamine. Doubling time in this medium is not significantly different from that in minimal medium. An enhancement of repression which partially mimics that observed on growth in medium supplemented with tryptophan plus casein hydrolysate is obtained when Ymel is grown on medium supplemented with tryptophan plus methionine. Threonine or phenylalanine plus tyrosine as separate medium supplements are independently capable of producing a 1.4-fold or 3.4-fold stimulation, respectively, but in combination only the phenylalanine plus tyrosine effect is manifested unless serine and glutamic acid or glutamine are included. Our data show that expression of the tryptophan biosynthetic enzymes can be significantly influenced in vivo as a result of growth in medium supplemented with a variety of amino acids.

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