Pathway choice in glutamate synthesis in Escherichia coli - PubMed (original) (raw)

Pathway choice in glutamate synthesis in Escherichia coli

R B Helling. J Bacteriol. 1998 Sep.

Abstract

Escherichia coli has two primary pathways for glutamate synthesis. The glutamine synthetase-glutamate synthase (GOGAT) pathway is essential for synthesis at low ammonium concentration and for regulation of the glutamine pool. The glutamate dehydrogenase (GDH) pathway is important during glucose-limited growth. It has been hypothesized that GDH is favored when the organism is stressed for energy, because the enzyme does not use ATP as does the GOGAT pathway. The results of competition experiments between the wild-type and a GDH-deficient mutant during glucose-limited growth in the presence of the nonmetabolizable glucose analog alpha-methylglucoside were consistent with the hypothesis. Enzyme measurements showed that levels of the enzymes of the glutamate pathways dropped as the organism passed from unrestricted to glucose-restricted growth. However, other conditions influencing pathway choice had no substantial effect on enzyme levels. Therefore, substrate availability and/or modulation of enzyme activity are likely to be major determinants of pathway choice in glutamate synthesis.

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Figures

FIG. 1

Comparison of GS levels during unlimited and glucose-limited growth. (Left) Total (upper) and unadenylylated (lower) enzyme activity. (Right) Average number of active monomers in the holoenzyme, as a function of dilution rate. The maximum specific growth rate is 0.44 h−1.

FIG. 2

Growth disadvantage of a gdhA mutant during glucose-limited growth as a function of ammonium or phosphate concentration. Ammonium (■) and phosphate (•) concentrations are represented as fractions of those in 1× medium (see Materials and Methods). Data are taken from reference .

FIG. 3

GS during glucose-limited growth as a function of ammonium or phosphate concentration. (Left panel) The uppermost curves (•, ■) show total enzyme activity from RH830 cells, and the lowermost curves (○, □) represent unadenylylated GS activity. (Right panel) Average number of active monomers in the 12-monomer holoenzyme. Ammonium (■, □) and phosphate (•, ○) concentrations are represented as fractions of those in 1× medium (see Materials and Methods).

FIG. 4

GS during unrestricted growth on glucose as a function of ammonium or phosphate concentration. (Left panel) The uppermost curves (•, ■) show total enzyme activity from RH830 and the lowermost curves (○, □) represent unadenylylated GS activity. (Right panel) Average number of active monomers in the holoenzyme. Ammonium (■, □) and phosphate (•, ○) concentrations are represented as fractions of those in 1× medium.

FIG. 5

The growth disadvantage of glucose-limited cells lacking GDH as a function of the concentration of the nonmetabolizable glucose analog α-methylglucoside.

FIG. 6

GS during glucose-restricted growth in the presence of the nonmetabolizable glucose analog αMG. (Left panel) Total (upper) and unadenylylated (lower) enzyme activity from strain RH830. (Right panel) average number of active monomers in the holoenzyme.

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Pathway choice in glutamate synthesis in Escherichia coli - PubMed (original) (raw)