Methionine synthesis in Neurospora. The isolation of cystathionine

Among artificially produced biochemical mutants of Neurospora, those which have lost the ability to synthesize methionine form the largest class. At the present writing 87 occurrences of the methionineless character have been observed in this laboratory following treatment of wild type spores with high frequency radiations (1) or mustard gas (2). Methionineless mutants differ from wild type Neurospora in that they fail to grow on a medium containing only sugar, inorganic salts, and biotin, but do grow if, in addition to these constituents, methionine is supplied. In many of the mutants failure of methionine synthesis results from a block in the reduction of sulfate, which, except for a trace of biotin, is the sole source of sulfur in the basal medium. These strains can utilize reduced forms of inorganic sulfur for growth, as well as methionine and other organic sulfur compounds. On the other hand, some of the mutants require organically bound sulfur for growth, an indication that in these strains the block in methionine synthesis comes at a later stage than sulfate reduction. Similar classes of methionine-requiring mutants have been reported in the mold Ophiostoma by Fries (3) and in Escherichia coli by Lampen et al. (4-6)

The d-amino acid oxidase of Neurospora

Among artificially produced mutants of the mold Neurospora have been found strains lacking the ability to synthesize specific amino acids (1, 2). In the course of biochemical and genetic studies of this group of mutants it was observed that some of the mutants, e.g. those deficient in methionine, leucine, and arginine,(1) are able to utilize racemic mixtures of the amino acids with the same efficiency as the l, or physiologically occurring, forms. In the cases of the leucine- and the methionine-requiring mutants it was also possible to show utilization of the ar-keto analogues. It thus appeared possible that the mode of conversion of the d to the l isomers consists in oxidative deammation, followed by resynthesis. A study was therefore undertaken to test the ability of Neurospora to oxidize the “unnatural” optical isomers of the amino acids. It was found that extracts of the mold contain a d-amino acid oxidase similar in its action to the d-amino acid oxidase of mammalian kidney and liver (3). This finding supports the above hypothesis for the conversion of the d- to the l-amino acids. Since it appears that the d-amino acid oxidase has not been previously described in fungi, a number of experiments were performed on the Neurospora enzyme, the results of which are reported here

Biochemical genetics

The field under review is growing so rapidly that it is impossible to cover more than a sampling of recent papers in the allotted space. Important subjects such as the genetics and chemistry of viruses and certain topics in bacterial genetics have had to be omitted, while others have not received the treatment they deserve. Studies of a primarily biochemical nature in which mutants have been employed as tools have been reviewed, as is customary, although it is recognized that their genetic interest lies chiefly in their providing materials for the further study of gene action

Cellular and extracellular siderophores of Aspergillus nidulans and Penicillium chrysogenum

Aspergillus nidulans and Penicillium chrysogenum produce specific cellular siderophores in addition to the well-known siderophores of the culture medium. Since this was found previously in Neurospora crassa, it is probably generally true for filamentous ascomycetes. The cellular siderophore of A. nidulans is ferricrocin; that of P. chrysogenum is ferrichrome. A. nidulans also contains triacetylfusigen, a siderophore without apparent biological activity. Conidia of both species lose siderophores at high salt concentrations and become siderophore dependent. This has also been found in N. crassa, where lowering of the water activity has been shown to be the causal factor. We used an assay procedure based on this dependency to reexamine the extracellular siderophores of these species. During rapid mycelial growth, both A. nidulans and P. chrysogenum produced two highly active, unidentified siderophores which were later replaced by a less active or inactive product--coprogen in the case of P. chrysogenum and triacetylfusigen in the case of A. nidulans. N. crassa secreted coprogen only. Fungal siderophore metabolism is varied and complex

The biological significance of the search for extraterrestrial life

Biological significance of genetic definition of life in studies of extraterrestrial lif

The ornithine cycle in Neurospora and its genetic control

It has been emphasized by Haldane (1) that for studies of intermediary metabolism "the new science of genetics furnishes a very powerful method." Such a method is founded upon the general premises that genes control many of the chemical reactions within an organism, and that gene mutations by blocking a reaction chain at various points may, in effect, resolve a metabolic process into some of its constituent stages. For instance, the genetics of such diseases as alcaptonuria and cystinuria have elucidated certain problems in human metabolic processes (2), and studies in the genetics of plant pigments have increased the knowledge of of the biochemistry of anthocyanins (3). But the study of metabolism by way of genetic differences in naturally occurring populations is limited not only by the low rate of mutation but also by the lethal character of most mutations of genes controlling vital functions. By increasing the mutation rate of an organism, through irradiation or otherwise, it is possible to create a number of genetic blocks at various steps in the syntheses of substances or in other processes of metabolism. The problem of preserving mutations ordinarily lethal has been met by Beadle and Tatum (4) in a general course of procedure developed around work with the ascomycetous mold Neurospora. The wild type of this organism is able to carry out all the syntheses essential to its normal growth and reproduction if biotin, inorganic salts, and a suitable source of carbon are available. Strains of Neurospora are irradiated with x-ray or ultraviolet rays on the assumption that mutations will be induced in genes controlling the syntheses of such substances as vitamins and amino acids. Mutant strains of this kind cannot grow on merely inorganic salts, sugar, and biotin, "minimal medium," but can be expected to grow if the product of the blocked synthesis is added to the minimal medium. From irradiated Neurospora there has been isolated in this laboratory a series of mutant strains which require for growth the presence of arginine in the culture medium. A study of the specific biochemical characteristics of members of this group of mutants has made it possible to demonstrate in Neurospora crassa an ornithine cycle similar to that proposed by Krebs and Henseleit (5) as occurring in mammalian liver, and to assign various steps in the cycle to the influence of particular single genes. To our knowledge the ornithine cycle has not previously been demonstrated in plants

The action of certain substituted phenols on marine eggs in relation to their dissociation

It has been shown by Clowes and Krahl (1, 2) that various substituted phenols as well as dinitrophenol increase the respiratory rate of marine eggs. Also, the highly interesting reversible block to cleavage, which they found to occur at the maximum of respiratory stimulation, is likewise exhibited. The different substances (nitro- and halo-phenols and cresols in particular) used were found to be active in different concentrations, and some attempt is made to relate the activity to molecular structure. The degree of dissociation of the phenolic OH is taken to be of no significance in their experiments. There has been some controversy concerning this question. Field, Martin and Field (3, 4) showed that in yeast the amount of respiratory stimulation by 2,4-dinitrophenol and by 4,6-dinitrocresol depends upon the concentration of the undissociated form present, similar calculated concentrations of undissociated DNP giving at different pH's the same stimulation. Citing their own experiments and those of Ehrenfest and Ronzoni (5) on yeast, De Meio and Barron (6), on the other hand, disagree with this conclusion

Experiments on the carboxylase of pea roots

It is known that vitamin B1 is a growth factor for numerous bacteria and fungi including the yeasts (see the summary in Koser and Saunders (1938)). It has also been demonstrated that vitamin B1 is essential for the growth of the isolated roots of higher plants (Bonner, 1937; Robbins and Bartley, 1937). Because of this general vitamin B1 requirement of living organisms, it would seem a priori probable that the vitamin plays a role in some basic cellular process. That this is indeed the case was shown conclusively by the work of Peters and coworkers (see Peters and O’Brien (1938)) and of Lohmann and Schuster (1937). The latter workers found that the prosthetic group of yeast carboxylase is vitamin B1 pyrophosphate. In the case of yeast, vitamin B1 is, then, a constituent of a respiratory enzyme and vitamin B1 pyrophosphate is hence commonly referred to as “cocarboxylase,” a terminology used throughout this paper. Although considerable information is available concerning the rôle of vitamin B1 as a growth factor for roots, there is little known about the carboxylase of such roots. The present work was undertaken with the hope of elucidating possible relationships between vitamin B1 and the carboxylase of pea roots

Growth inhibition of neurospora by canavanine, and its reversal

Canavanine, an amino acid from jack beans, was discovered by Kitagawa and coworkers in 1929 (1, 2). The substance is not combined in the proteins of the seed, but occurs in the free state, and makes up 2.5 per cent of the dry weight of jack beans (3). In a series of papers available to the authors for the most part in abstract only, the Japanese workers have reported extensive investigations into the chemistry and physiology of the substance. The structure of canavanine was established by Gulland and Morris (4) and by Kitagawa and Takani (5) as NH2•C(:NII)•NII•O•CH2•CH2•CHNH2•COOH. Natural canavanine is of the L configuration (6)

Physiological Aspects of Genetics

A considerable amount of evidence indicates that desoxyribonucleic acid is capable of duplicating itself, a property also possessed by genes. (By a self-duplicating material, we mean one which plays some essential role in its own production.) Watson & Crick (1) have proposed a new structure for desoxyribonucleic acid which not only takes into account the existing analytical and x-ray diffraction data but also seems capable of explaining the mechanism of duplication. Their model consists of two helical chains coiled around the same axis, the purine and pyrimidine bases on the inside, the phosphate groups on the outside. The chains are held together by hydrogen bonds between the bases, the adenine residues of either chain being bonded specifically to thymine in the other, and similarly guanine to cytosine. The sequence of bases along one chain is not restricted, but once fixed the sequence along the other chain is determined. This complementarity, which is the most novel feature of the structure, suggests that duplication takes place by separation of the two chains, followed by the synthesis of its complement alongside each chain. The model is supported by recent x-ray diffraction studies (2, 3)