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D M Downs

Publications and source records attributed to D M Downs.

7 recordsLinked to original sources
PubMed1992

Evidence for a new, oxygen-regulated biosynthetic pathway for the pyrimidine moiety of thiamine in Salmonella typhimurium.

The synthesis of the pyrimidine moiety of thiamine (vitamin B1) shares five reactions with the de novo purine biosynthetic pathway. Aminoimidazole ribotide (AIR) is the last common intermediate before the two pathways diverge. Evidence for the existence of a new pathway to the pyrimidine which bypasses the de novo purine biosynthetic pathway is reported here. This pathway is only expressed under anaerobic growth conditions and is denoted alternative pyrimidine biosynthesis or APB. Labeling studies are consistent with pantothenate being a precursor to the pyrimidine moiety of thiamine that is synthesized by the APB pathway. The APB pathway is independent of the alternative purF function which was proposed previously (D. M. Downs and J. R. Roth, J. Bacteriol. 173:6597-6604, 1991). The alternative purF function is shown here to be affected by temperature and exogenous pantothenate. Although the evidence suggests that the APB pathway is separate from the alternative purF function, the relationship between this function and the APB pathway is not yet clear.

Anaerobiosis↗
PubMed1991

Synthesis of thiamine in Salmonella typhimurium independent of the purF function.

In Salmonella typhimurium, the first five steps in purine biosynthesis also serve as the first steps in the biosynthesis of the pyrimidine moiety of thiamine (vitamin B1). Strains with null mutations of the first gene of purine-thiamine synthesis (purF) can, under some circumstances, grow without thiamine. This suggests the existence of an alternative pathway to thiamine that can function without the purF protein. To demonstrate the nature and map position of the purF mutations corrected, a fine-structure genetic map of the purF gene was made. The map allows identification of deletion mutations that remove virtually all of the purF gene, as defined by mutations. We describe conditions and mutations (panR) which allow B1 synthesis appears to require enzymes which act mutants lacking purF function. The alternative route of B1 synthesis appears to require enzymes which act subsequent to the purF enzyme in the purine pathway.

Adenine↗
PubMed1990

Synthesis of the iron-molybdenum cofactor of nitrogenase is inhibited by a low-molecular-weight metabolite of Klebsiella pneumoniae.

The in vitro synthesis of the iron-molybdenum cofactor nitrogenase was inhibited by a low-molecular-weight factor. This inhibitory factor was present in the membrane extracts of wild-type and nif mutant strains of Klebsiella pneumoniae that were grown under conditions that either repressed or derepressed nitrogenase expression. In vitro, the inhibition was specific for the NifB protein. Addition of this factor to K. pneumoniae cells at various times during nif derepression decreased nitrogenase activity, presumably through inhibition of iron-molybdenum cofactor synthesis. The inhibitor was purified by solvent extraction and chromatography on DEAE-cellulose, silica gel, and aluminum oxide columns.

Bacterial Proteins↗
PubMed1987

A novel P22 prophage in Salmonella typhimurium.

Under several sets of conditions, all of which seem to perturb purine metabolism, Salmonella typhimurium releases a variety of phages which were not known to be present in the strain. These cryptic phages are not induced by UV irradiation. Furthermore, the induction process does not require a functional recA gene product. While phages of several phenotypic classes have been recovered, including both turbid and clear plaque formers, all appear to be variants of P22 because all show DNA restriction patterns indistinguishable from that of P22. The variety of types suggests that the cryptic prophage is mutagenized as a consequence of the induction process. All the temperature phages tested are capable of transducing a variety of chromosomal markers with high efficiency. The phages induced in this novel way are capable of forming plaques on the strains that gave rise to them. Since the strains releasing phage are not immune to P22, the parental lysogens must not express immunity and the phage must be held in a cryptic state by a novel mechanism. The released phage possess an intact P22 immunity system because many can form standard immune lysogens after reinfection of Salmonella. These results raise the possibility that Salmonella typhimurium harbors cryptic phages that are subject to a novel system of global control related to purine metabolism. Preliminary evidence suggests that the regulation system may involve DNA modification.

Anaerobiosis↗