5-Hydroxy-L-norvaline

We show that a tktA tktB double mutant, which is devoid of the two known transketolase isoenzymes of Escherichia coli K-12, requires pyridoxine (vitamin B6) as well as the aromatic amino acids and vitamins for growth. This pyridoxine requirement can also be satisfied by 4-hydroxy-L-threonine or glycolaldehyde. These results provide direct evidence that D-erythrose-4-phosphate is a precursor of the pyridine ring of pyridoxine. In addition, they show that the two major E. coli transketolase isoenzymes are not required for the biosynthesis of D-1-deoxyxylulose, which is thought to be another precursor of pyridoxine.

Pyridoxineless mutants of Escherichia coli are lysed in a few hours when starved for pyridoxine in a glucose minimal medium containing glycine at 10 mM. The lysis is prevented equally well by l-alanine and by d-alanine when either is present at 0.1 mM. The lysis is potentiated by 0.5 mM l-methionine. The peculiar susceptibility of E. coli B to glycine-mediated lysis during starvation for pyridoxine suggests that the starvation reduces the availability of some normal antagonist of glycine, presumably alanine.

Dempsey, Walter B. (University of Florida, Gainesville). Control of pyridoxine biosynthesis in Escherichia coli J. Bacteriol. 90:431-437. 1965.-The total pyridoxine in a culture of exponentially growing Escherichia coli was 3.6 x 10(-10) moles per mg of dry cells. One-fourth of this total was present in the medium, and was at least 90% pyridoxal 5'-phosphate. Both pyridoxol and pyridoxal, when present initially at 6 x 10(-7)m, substituted entirely for de novo synthesis of pyridoxine. The other four forms of the pyridoxine group were ineffective at this concentration. Pyridoxine biosynthesis in exponentially growing cultures of E. coli was immediately stopped by adding pyridoxol to the medium to a final concentration of 4 x 10(-7)m. Amino acid auxotrophs of E. coli suspended in minimal medium without the required amino acid produced extracellular pyridoxine for several hours at an average rate of 1.3 x 10(-10) moles per hr per mg of dry cells. The rate of production of extracellular pyridoxine by a threonine-starved threonine auxotroph was not altered by growing the culture before starvation in media containing 6 x 10(-5)m pyridoxol. The extracellular form of pyridoxine produced in these starvations required hydrolysis to be detected in the bioassay used in these measurements. The total findings suggest that pyridoxine biosynthesis is controlled by a rapidly acting mechanism but not by repression.