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Biomedical subjects

R H Davis

Publications and source records attributed to R H Davis.

At least 289 records · Page 16Linked to original sources
PubMed1968

Mutants of Neurospora crassa deficient in ornithine-delta-transmainase.

The arg-12(s) mutation of Neurospora causes a partial block in the ornithine transcarbamylase (OTC) reaction. Strains carrying this mutation will use endogenous ornithine, but not exogenous ornithine, as a precursor of arginine. Certain strains carrying arg-12(s) may be used for direct selection of variants able to use exogenous ornithine as an arginine precursor. Among eight such derivatives, six lacked the catabolic enzyme ornithine transaminase (OTA). All six mutations were alleles of a single gene, designated ota, on linkage group III. No mutation affected arginase, the first enzyme in the catabolic pathway with OTA. Strains carrying ota mutations alone are vigorous prototrophs, but, unlike wild-type Neurospora, fail to use ornithine efficiently as a sole nitrogen source. The selection method by which ota mutants arose suggests that OTA is severely competitive with OTC for exogeneous ornithine.

Arginase↗
PubMed1968

Utilization of exogenous and endogenous ornithine by Neurospora crassa.

Through the use of a mutant deficient in ornithine-delta-transaminase (OTA), it is shown that this enzyme normally has no obligate or even major biosynthetic role in Neurospora. The pathways of ornithine and proline synthesis proceed wholly independently of each other in OTA-less strains. It is probable that OTA functions as an enzyme of arginine catabolism. With mutants affected in OTA, ornithine transcarbamylase, and the synthesis of ornithine, it was demonstrated that exogenous and endogenous ornithine are utilized in different ways. Exogenous ornithine is destined mainly for catabolism, whereas endogenous ornithine is destined mainly for biosynthesis. It is suggested that this distinction depends upon differences in the intracellular location or origin of the two sources of ornithine.

Arginine↗
PubMed1966

A methane-dependent coccus, with notes on classification and nomenclature of obligate, methane-utilizing bacteria.

Foster, J. W. (The University of Texas, Austin), and Richard H. Davis. A methane-dependent coccus, with notes on classification and nomenclature of obligate, methane-utilizing bacteria. J. Bacteriol. 91:1924-1931. 1966.-A new coccus-shaped bacterium capable of aerobic growth at the expense of methane or methanol in a mineral salts medium is described. The organism did not grow at the expense of any of the conventional substrates or homologous hydrocarbons tested. It is gram-negative, nonmotile, and thermotolerant. It grows well at 50 C, optimally at 37 C, but does not grow at 55 C. The cells are encapsulated and have a characteristic diplococcoid arrangement. Washed, "resting-cell" suspensions oxidized certain primary alcohols and short-chain alkanes, an example of "nongrowth oxidation." Of the methane-C utilized, 86% was "fixed" in organic form; the remainder was oxidized to CO(2). The guanine-cytosine content of the extracted deoxyribonucleic acid was 62.5%. Obligate methane-utilizing bacteria are considered as "one-carbon" organisms rather than hydrocarbon utilizers. The assimilation pathway in the obligate methane-methanol bacteria is different from that in the facultative methanol utilizers. Nomenclatural problems arising from the use of the prefix "Methano-" to denote both bacteria that oxidize methane and bacteria that produce methane are discussed. The obligate, one-carbon, methane-methanol bacteria are considered as "methyl" utilizers, and the prefix "Methylo-" is suggested as a solution to the problem of generic cognomens. "Methylococcus capsulatus" gen. n., sp. n. is the name proposed for the new methane coccus.

Bacteria↗