Metabolic Relations between l-Threonine and Glycine in Escherichia coli

Inactivation of Escherichia coli 2-amino-3-ketobutyrate CoA ligase by phenylglyoxal and identification of an active-site arginine peptide

Treatment of homogeneous preparations of 2-amino-3-ketobutyrate CoA ligase from Escherichia coli, a pyridoxal 5'-phosphate-dependent enzyme, with phenylglyoxal, 4-(oxyacetyl)phenoxyacetic acid, 2,3-butanedione, or 1,2-cyclohexanedione results in a time- and concentration-dependent loss of enzymatic activity. Phenylglyoxal in 50 mM phosphate buffer (pH 7.0) is the most effective modifier, causing > 95% inactivation within 20 min at 25 degrees C. Controls establish that this inactivation is not due to modifier-induced dissociation or photoinduced nonspecific alteration of the ligase. The substrate, acetyl CoA, or the coenzyme, pyridoxal 5'-phosphate, gives > 50% protection against inactivation. Enzyme partially inactivated by phenylglyoxal has the same Km value for glycine but the Vmax decreases in proportion to the observed level of inactivation. Whereas the native apoligase shows good recovery of activity with time in parallel with an increase in 428-nm absorptivity when incubated with pyridoxal 5'-phosphate, no such effects are seen with the phenylglyoxal-modified apoligase. Reaction of the enzyme with [14C]phenylglyoxal allowed for the isolation of a peptide which, by amino acid composition and sequencing data, was found to correspond to residues 349-378 in the intact enzyme. These results indicate that arginine residue-366 and/or residue-368 in the primary structure of E. coli 2-amino-3-ketobutyrate ligase is at the active site.

Genetic characterization of a highly efficient alternate pathway of serine biosynthesis in Escherichia coli

There exists in Escherichia coli a known set of enzymes that were shown to function in an efficient and concerted way to convert threonine to serine. The sequence of reactions catalyzed by these enzymes is designated the Tut cycle (threonine utilization). To demonstrate that the relevant genes and their protein products play essential roles in serine biosynthesis, a number of mutants were analyzed. Strains of E. coli with lesions in serA, serB, serC, or glyA grew readily on minimal medium supplemented with elevated levels of leucine, arginine, lysine, threonine, and methionine. No growth on this medium was observed upon testing double mutants with lesions in one of the known ser genes plus a second lesion in glyA (serine hydroxymethyltransferase), gcv (the glycine cleavage system), or tdh (threonine dehydrogenase). Pseudorevertants of ser mutants capable of growth on either unsupplemented minimal medium or medium supplemented with low levels of leucine, arginine, lysine, threonine, and methionine were isolated. At least two unlinked mutations were associated with such phenotypes.

Derivation of glycine from threonine in Escherichia coli K-12 mutants

Escherichia coli AT2046 has been shown previously to lack the enzyme serine transhydroxymethylase and to require exogenous glycine for growth as a consequence. Strains JEV73 and JEV73R, mutants derived from strain AT2046, are shown here to be serine transhydroxymethylase deficient, but able to derive their glycine from endogenously synthesized threonine. Leucine is shown to be closely involved in the regulation of biosynthesis of glycine, to spare glycine in strain AT2046T, to replace glycine in strain JEV73, and to increase threonine conversion to glycine in a representative prototroph of E. coli. An interpretation of strains JEV73 and JEV73R as regulatory mutants of strain AT2046 is given. A hypothesis as to the role of leucine as a signal for nitrogen scavenging is suggested.

Mutations Influencing the Assimilation of Nitrogen by Yersinia pestis

Cells of 20 isolates of Yersinia ( Pasteurella ) pestis exhibited an unusual nutritional requirement which could be fulfilled by glycine or l -threonine. Meiotrophic mutants which required neither of these amino acids (Gly/Thr + ) were isolated from cultures of all 20 strains at a frequency of 10 −7 . Wild-type and Gly/Thr + cells of 14 strains failed to utilize l -amino acids or urea (0.01 m ) as primary sources of nitrogen and grew slowly in the presence of low concentrations of NH 4 + (≦ 5 m m ). Cells of six strains (termed N + ) utilized certain l -amino acids and urea (0.01 m ) as primary sources of nitrogen and grew rapidly in the presence of ≦ 5 m m NH 4 + . N + but not N − organisms cultivated with NH 4 + (0.01 m ) as a primary source of nitrogen excreted a complete spectrum of naturally occurring amino acids; under this condition of growth the aspartase and particulate nicotinamide adenine dinucleotide phosphate transhydrogenase activities of N + and N − cells were repressed. N + meiotrophs arose at a frequency of 10 −6 in cultures of all 14 N − isolates, and urease-positive meiotrophs could be selected at a frequency of 10 −7 from N + but not N − cells of all 20 strains on a medium containing urea (0.01 m ) as a primary source of nitrogen. These findings illustrate a reversible loss of genetic potential which has occurred during the evolution of Y. pestis as an obligate parasite and suggest that this organism is unable to efficiently remove dispensable deoxyribonucleic acid from its chromosome.