Nutritional Auxotrophs

The ability to isolate the many auxotrophic (nutritionally dependent) strains of Escherichia coli and Aerobacter aerogenes, with which Davis elucidated several pathways in biosynthesis, depended on his use of the observation that only under conditions which allow growth to take place does penicillin kill bacterial cells. Essentially the same method was developed, simultaneously and independently, by Lederberg and Zinder. ... 

The introduction of techniques for mutagenesis by UV irradiation or by the use of chemicals considerably extended the applications of microbial studies to nutrition (Davis, 1954-1955). Auxotrophic mutants were produced with nutrient dependencies not shown in the untreated parental strains (Beadle and Tatum, 1940). The fortuitous discovery of penicillin by Fleming and its successful use in the treatment of infections (Florey) promoted exhaustive research into its mode of action. Eventually it was established that penicillin prevented the proliferation of gram-positive bacteria by blocking the synthesis of their cell walls... 

In later studies, with cell-free extracts, the enzyme responsible for the dehydration of 5-dehydroquinate to 5-dehydroshikimate (5-dehydroquinase) was partially purified and was characterized." It was highly specific for this reaction, being unable to dehydrate the closely related quinic acid to shikimic acid. As was to be expected, 5-dehydroguinase was absent from extracts of quintuple auxotrophs which, on nutritional grounds (see Section... 

On the basis of nutritional requirements and accumulation patterns of multiple aromatic auxotrophs, the relationships that emerge are entirely... 

Certain phenylalanine auxotrophs of E. coli were observed to possess the unusual ability of developing a growth factor for themselves. The accumulation of a nutritionally inactive compound, its spontaneous decomposition to phenylpyruvic acid (as the pH of the growth medium dropped), and the conversion of the latter to phenylalanine, were shown to be responsible for this effect. The unstable compound, prephenic acid, was isolated from culture filtrates and was characterized as XVAlthough its half-life at room temperature in N hydrochloric acid was 1 noinute, prephenic acid was relatively stable at neutral pH. Its enzymic conversion... 

In studying the biochemical genetics of tryptophan synthesis in Pseudomonas acidovorans, Buvinger et al.6 isolated 60 independent tryptophan auxotrophs of Pseudomonas acidovorans and characterized them for nutritional response to intermediates of the pathway, accumulation of intermediates, and levels of tryptophan-synthetic enzymes. Mutants for each of the seven proteins catalyzing the five steps of tryptophan synthesis were... 

Nutritional auxotrophs can be described as conditionally lethal mutants they survive only if the medium is supplemented with the nutrient, whose S5mthesis depends upon the missing enzyme. Other kinds of conditional lethal mutations permit study of almost every gene in an organism. For example, temperature-sensitive (fs) mutants grow perfectly well at a low temperature, e.g., 25°C, but do not grow at a higher temperature, e.g., Many... 

---