CONTAMINATION OF COMMERCIAL L-LEUCINE PREPARATIONS WITH METHIONINE AND CYSTINE

Aspartokinase activity and the developmental cycle of Myxococcus xanthus

The relationship between aspartokinase activity and fruiting body formation in Myxococcus xanthus was investigated. Two required amino acids, methionine and isoleucine, which stimulated the enzyme in vitro also inhibited fruiting body formation when added to 0.1% Casitone agar. Threonine, a potent feedback inhibitor of the aspartokinase, completely reversed the effects of methionine and isoleucine both on enzyme activity and fruiting body formation. A mutant, M. xanthus FB-S, which had the unusual property of forming fruiting bodies on 1.0% Casitone agar, also exhibited an altered regulation of aspartokinase activity. Spermidine, which is a strong stimulator of the enzyme in vitro, interfered with the developmental cycle of both M. xanthus FB and FS-S. During glycerol induction of myxospores the level of aspartokinase dropped more than 75% during the first hour. These data indicate a strong correlation between aspartokinase activity and the induction of the developmental cycle in M. xanthus. It is suggested that the decrease in aspartokinase activity results in diaminopimelic acid starvation, blockage of cell wall growth, and subsequent induction of the developmental cycle.

Genetic studies of sulfadiazine-resistant and methionine-requiring Neisseria isolated from clinical material

Deoxyribonucleate (DNA) preparations were extracted from Neisseria meningitidis (four isolates from spinal fluid and blood) and N. gonorrhoeae strains, all of which were resistant to sulfadiazine upon primary isolation. These DNA preparations, together with others from in vitro mutants of N. meningitidis and N. perflava, were examined in transformation tests by using as recipient a drug-susceptible strain of N. meningitidis (Ne 15 Sul-s Met(+)) which was able to grow in a methionine-free defined medium. The sulfadiazine resistance typical of each donor was introduced into the uniform constitution of this recipient. Production of p-aminobenzoic acid was not significantly altered thereby. Transformants elicited by DNA from the N. meningitidis clinical isolates were resistant to at least 200 mug of sulfadiazine/ml, and did not show a requirement for methionine (Sul-r Met(+)). DNA from six strains of N. gonorrhoeae, which were isolated during the period of therapeutic use of sulfonamides, conveyed lower degrees of resistance and, invariably, a concurrent methionine requirement (Sul-r/Met(-)). The requirement of these transformants, and that of in vitro mutants selected on sulfadiazine-agar, was satisfied by methionine, but not by vitamin B(12), homocysteine, cystathionine, homoserine, or cysteine. Sul-r Met(+) and Sul-r/Met(-) loci could coexist in the same genome, but were segregated during transformation. On the other hand, the dual Sul-r/Met(-) properties were not separated by recombination, but were eliminated together. DNA from various Sul-r/Met(-) clones tested against recipients having nonidentical Sul-r/Met(-) mutant sites yielded Sul-s Met(+) transformants. The met locus involved is genetically complex, and will be a valuable tool for studies of genetic fine structure of members of Neisseria, and of genetic homology between species.