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Biomedical subjects

H K Mitchell

Publications and source records attributed to H K Mitchell.

At least 55 records · Page 3Linked to original sources
PubMed1969

Mutants affecting thymidine metabolism in Neurospora crassa.

When (14)C-thymidine labeled only in the ring is administered to Neurospora crassa, the majority of the recovered label is found in the ribonucleic acid (RNA). Three mutants were isolated in which different steps are blocked in the pathway that converts the pyrimidine ring of thymidine to an RNA precursor. Evidence from genetic, nutritional, and accumulation studies with the three mutants shows the pathway to proceed as follows: thymidine --> thymine --> 5-hydroxymethyluracil --> 5-formyluracil --> uracil --> uridylic acid. A mutant strain in which the thymidine to thymine conversion is blocked is unable to metabolize thymidine appreciably by any route, including entry into nucleic acids. This suggests that Neurospora lacks a thymidine phosphorylating enzyme. A second mutation blocks the pathway at the 5-hydroxymethyluracil to 5-formyluracil step, whereas a third prevents utilization of uracil and all compounds preceding it in the pathway. The mutant isolation procedures yielded three other classes of mutations which are proposed to be affecting, respectively, regulation of the thymidine degradative pathway, transport of pyrimidine free bases, and transport of pyrimidine nucleosides.

Carbon Isotopes↗
PubMed1967

Characteristics of Tphi3, a bacteriophage for Bacillus stearothermophilus.

A bacteriophage (Tphi3) which infects the thermophilic bacterium Bacillus stearothermophilus ATCC 8005 was isolated and characterized. Infection of the bacterium by the bacteriophage was carried out at 60 C, the optimal growth temperature of the host. At 60 C, the phage had a latent period of 18 min and a burst size of about 200. The phage was comparatively thermostable in broth. The halflife of Tphi3 was 400 min at 60 C, 120 min at 65 C, 40 min at 70 C, and 12 min at 75 C. The activation energy for the heat inactivation of Tphi3 was 56,000 cal. The buoyant density of Tphi3 in a cesium chloride density gradient was 1.526 g/ml. Electron micrographs of Tphi3 indicate that the phage has a head that is 57 mmu long. The dimensions and shape of the head are compatible with those of a regular icosahedron. Each edge of the head is 29 mmu long. The tail of Tphi3 is 125 mmu long and 10 mmu wide. There are about 30 cross-striations that are spaced at 3.9-mmu intervals along the tail. Under the conditions investigated, Tphi3 adsorbed slowly to the host. Only 2.8% of the phage adsorbed in 10 min at 60 C, the normal incubation temperature that was used. Tphi3 was not infective to four other thermophilic strains or to two mesophilic strains of bacteria.

Bacillus↗