Intermediates in amino acid biosynthesis

Mutants of Salmonella typhimurium lacking phosphoenolpyruvate carboxykinase and alpha-ketoglutarate dehydrogenase activities

Two auxotrophic mutants (SM16 and SM51) of Salmonella typhimurium, which for aerobic growth, with hexoses as carbon source, required lysine and methionine (SM51 required also nicotinic acid), were isolated and characterized. The requirement for the amino acids disappeared in anaerobiosis. Neither lipoate nor 4-hydroxybenzoate was effective in supporting aerobic growth of the mutants. The lysine and methionine requirement for aerobic growth was due to the absence in the mutants of the enzymatic activities of the alpha-ketoglutarate dehydrogenase complex. The mutants could not use succinate as carbon source even after enrichment of the growth medium with acid-hydrolyzed casein and yeast extract. No phosphoenolpyruvate carboxykinase activity was found in the mutants, a phenomenon which explained their inability to use succinate. By interrupted conjugation and by transduction experiments, the positions of the three affected loci, pck, suc, and Nic, were located at approximately 17 to 19 min of the S. typhimurium chromosome; they were found to be closely linked. From different criteria, it appears as if the genetic lesions present in both mutants are due to deletion of a small chromosome fragment.

Mutants of Neurospora crassa deficient in ornithine-delta-transmainase

The arg-12(s) mutation of Neurospora causes a partial block in the ornithine transcarbamylase (OTC) reaction. Strains carrying this mutation will use endogenous ornithine, but not exogenous ornithine, as a precursor of arginine. Certain strains carrying arg-12(s) may be used for direct selection of variants able to use exogenous ornithine as an arginine precursor. Among eight such derivatives, six lacked the catabolic enzyme ornithine transaminase (OTA). All six mutations were alleles of a single gene, designated ota, on linkage group III. No mutation affected arginase, the first enzyme in the catabolic pathway with OTA. Strains carrying ota mutations alone are vigorous prototrophs, but, unlike wild-type Neurospora, fail to use ornithine efficiently as a sole nitrogen source. The selection method by which ota mutants arose suggests that OTA is severely competitive with OTC for exogeneous ornithine.

Aromatic amino acid biosynthesis: gene-enzyme relationships in Bacillus subtilis

Single step mutants of Bacillus subtilis which required either one or all of the aromatic amino acids for growth were isolated. The relevant gene defect was determined for each mutant by enzyme assays in vitro. A mutant deficient in each enzyme step of aromatic amino acid biosynthesis was found with the exceptions of the shikimate kinase and the phenylalanine and tyrosine transaminases. Representative mutants carrying the defective genes were mapped by deoxyribonucleic acid mediated transformation by reference to the aromatic amino acid gene (aro) cluster and, alternately, to any of the other unlinked aro genes. The genes coding for dehydroquinate synthetase, 3-enol pyruvylshikimate 5-phosphate synthetase, one form of chorismate mutase, and prephenate dehydrogenase are linked to the aro cluster. Except for the previously identified linkage between the genes of 3-deoxy-d-arabino heptulosonic acid 7-phosphate synthetase and one species of chorismate mutase, the other genes involved in this pathway are neither linked to the aro cluster nor to each other.

The relationship of 4-hydroxybenzoic acid to lysine and methionine formation in Escherichia coli

1. A multiple aromatic mutant, Escherichia coli 156:53D2, required 4-hydroxybenzoic acid for rapid aerobic growth on a number of carbon sources. 2. In the absence of 4-hydroxybenzoic acid aerobic growth was stimulated by a mixture of lysine and methionine and by succinate. The influence of the amino acids is attributed to a sparing of succinyl-CoA. 3. Low activities of both alpha-oxoglutarate dehydrogenase and fumarate reductase were found in organisms grown aerobically without 4-hydroxybenzoate and consequently both mechanisms known for the formation of succinate were impaired. 4. The low fumarate-reductase activity in these organisms was due to repression of enzyme synthesis by aeration and not to enzyme inactivation. In contrast lactate dehydrogenase and ethanol dehydrogenase were induced. This is interpreted as the appearance of alternative routes of NADH oxidation when electron transfer to oxygen is impaired. 5. The activities of the other tricarboxylic acid-cycle enzymes tested were little influenced by 4-hydroxybenzoate deficiency, although anaerobiosis resulted in a fall in activity.

5-Dehydroshikimate reductase in the tea plant (Camellia sinensis L.). Properties and distribution

1. A method for the extraction of 5-dehydroshikimate reductase (EC 1.1.1.25) from tea plant tissues in an active soluble state has been developed. It is dependent on the use, in the extraction medium, of an insoluble polyphenol adsorbent (Polyclar AT), which prevents the polyphenols present from precipitating all the proteins. 2. The enzyme has the following properties: pH optima at pH10.1 in glycine-sodium hydroxide buffer and at pH7.7 in tris-hydrochloric acid buffer; K(m) (NADP) 32mum and K(m) (shikimate) 0.43mm; and NADP-specificity. It was completely inhibited by 0.33mm-p-chloromercuribenzoate and this inhibition was completely reversed by 10mm-cysteine. Iodoacetate and arsenite inhibited the enzyme to a smaller extent. 3. The specific activity of the enzyme was higher in the parts of the actively growing shoot tips (third leaf>stem>second leaf>first leaf>bud) than in the mature leaves. However, the mature leaves had the greatest total activity. 4. The importance of these findings with respect to flavanol biosynthesis in tea plants is discussed.