Recombination analysis of naturally diploid Candida albicans

A multiply auxotrophic strain, hOG45, was derived from Candida albicans ATCC 10261. Prototrophic revertants of this multiple auxotroph were selected after mutagenesis. These prototrophic revertants were distinguishable from the original prototroph, ATCC 10261, because of their mitotic instability. They gave rise to auxotrophic derivatives which displayed one or more of the auxotrophic requirements characteristic of hOG45. Two of the auxotrophic requirements, those for adenine and methionine, frequently reappeared together in the auxotrophic derivatives of the prototrophic revertants. This apparent linkage of ade and met was confirmed by protoplast fusion analysis of the original auxotroph. These data indicate that C. albicans ATCC 10261 is diploid, the multiple auxotroph h0G45 is homozygous for recessive auxotrophic alleles, the prototrophic revertants are multiple heterozygotes, the auxotrophic derivatives are homozygotes produced by mitotic crossing-over, and the association between the ade and met alleles is due to linkage.

Genetic analysis of Candida albicans: identification of different isoleucine-valine, methionine, and arginine alleles by complementation

By using the spheroplast fusion technique as a tool for genetic analysis, we have demonstrated complementation among three of four isoleucine-valine mutants, two of three methionine mutants, and two arginine mutants of independent origin from two different Candida albicans isolates. The two adenine mutants derived from the same parent strain did not complement. Complementation resulted predominantly from heterokaryon formation and, in some cases, from heterozygote formation. In either case, most fusion products were unstable and showed nuclear as well as chromosomal segregation, in a few cases resulting in recombination of parental auxotrophic markers. However, some fusion products were fairly stable.

De novo biosynthesis of nucleotides and of nucleic acids in different regions of developing rat brain: effect of undernutrition

The effect of undernutrition on the de novo biosynthesis of nucleic acids in cerebral hemispheres, cerebellum, and brain stem of rats at different days of postnatal development was studied. The experiments were carried out in vitro by measuring the incorporation of [14C]-formate into the adenine nucleotide of the acid-soluble fraction of RNA and DNA, as well as into the thymine of DNA. The results obtained indicate that undernutrition during fetal and postnatal development impairs the de novo synthesis of the purine nucleotides of RNA and DNA at 5 days of age and delays it thereafter in the various brain regions examined, particularly in the cerebellum.

Nonenzymatic methylation of DNA by S-adenosylmethionine in vitro

S-Adenosylmethionine was found to methylate DNA non-enzymatically to produce the same putative promutagenic and procarcinogenic lesions formed by carcinogenic chemical methylating agents. The formation of 7-methylguanine was confirmed by u.v. spectrophotometry, the formation of O6-methylguanine and 3-methyladenine was suggested by the cochromatography of radioactivity with standard bases. It is possible that this reaction may explain the presence of constitutive cellular enzymes specifically for the repair of methylated DNA, and may indicate the mechanism whereby methylated guanine is formed in the liver DNA of rats with chemically induced liver damage.

Dependence of adenine production upon polyamine synthesis in cultured human lymphoblasts

The exact source of de novo adenine produced by mammalian cells remain poorly understood, and this prompted the present study. Using a human lymphoblastoid cell line (WI-L2) deficient in adenine phosphoribosyltransferase (EC 2.4.2.7), we have quantitated the rate of adenine synthesis and the relative importance of the phosphorolysis of 5'-methylthioadenosine versus adenosine or 2'-deoxyadenosine in adenine generation. Dividing adenine phosphoribosyltransferase-deficient WI-L2 cells produced adenine at a rate of 0.27 nmol/mg protein/h. This represented approximately 10% of the rate of hypoxanthine production by WI-L2 cells deficient in hypoxanthine phosphoribosyltransferase (EC 2.4.2.8) but was equivalent to the rate of 5'-methylthioadenosine synthesis by human lymphoblastoid CCRF-CEM deficient in 5'-methylthioadenosine, phosphorylase (5'-methylthioadenosine: orthophosphate methylthioribosyltransferase). Up to 97% of adenine, but not hypoxanthine, synthesis was inhibited dose-dependently by the S-adenosylmethionine decarboxylase-inhibitor methylglyoxal bis(guanylhydrazone) and also by spermidine and spermine, but was enhanced by putrescine. The addition of 2-fluoroadenine, a potent competitive inhibitor of methylthioadenosine phosphorylase (Ki = 0.43 microM) to adenine phosphoribosyl-transferase-deficient cells resulted in a progressive accumulation of 5'-methylthioadenosine in the culture medium, and up to an 85% decrease in adenine production at non-toxic concentrations. These results show that de novo adenine synthesis by dividing human cells is considerable, and that 85-97% derives from the cleavage of 5'-methylthioadenosine and hence from polyamine synthesis.

Repair of O6-methylguanine in rat liver DNA is enhanced by pretreatment with single or multiple doses of aflatoxin B1

Pretreatment of rats by the repeated administration of certain alkylating carcinogens has been shown to stimulate the removal of O6-alkylguanine from hepatic DNA. Prolonged feeding with the aromatic amide 2-acetylaminofluorene has a similar effect. In this report, aflatoxin B1, an agent from another chemically distinct class of carcinogen, is shown to be capable of stimulating the repair of O6-methylguanine in hepatic DNA. The sensitivity of this system is shown by the fact that this repair response can be fully stimulated as early as 1 day after treatment with a single dose.

Allele specific and locus non-specific suppressors in Aspergillus nidulans

Using N-methyl-N'-nitro-N-nitrosoguanidine, ultraviolet irradiation, ethyl methanesulphonate or 4-nitroquinoline-1-oxide mutagenesis and an enrichment method for the isolation of auxotrophs, 25 mutants with defects in the adA locus were obtained after screening 41,376 colonies. One of these, adA24, did not complement with any of the other adA mutants, had a very high reversion rate and had some other properties which usually characterize strains carrying nonsense mutations. All revertants of adA24 carried dominant suppressor mutations. A group of adA24 suppressors was tested for allele and locus specificity. They were found to suppress only some adA alleles, and at the same time, some mutations in the methG, methH, argB and proA loci. It is proposed that the allele specific and locus non-specific adenine suppressors are suppressors of nonsense mutations.

Purine metabolism in human lymphocytes

In peripheral human blood lymphocytes the uptake and metabolism of adenine, guanine, and hypoxanthine was investigated. This was achieved by incubation of purified lymphocytes with 14C-purine bases, separation of cells from the incubation medium by a rapid filtration technique, and subsequent separation of the acid soluble material by thin-layer chromatography. No perferential uptake for one of the purine bases was observed. In all cases only traces of 14C-purine bases not added originally and labeled nucleosides could be demonstrated. Approximately 2/3 of adenine and 1/2 of guanine or hypoxanthine were converted to nucleotides. Separation of formed nucleotides showed that adenine and guanine were metabolized mainly to their corresponding nucleotides; hypoxanthine was converted to a considerable amount to adenine nucleotides and only to a small proportion into its own nucleotides. These results demonstrate the predomonance of adenine nucleotide formation in normal human lymphocytes.

Allelism of methionine-sensitive mutants of Schizosaccharomyces pombe to loci involved in adenine biosynthesis

Methionine-sensitive mutants ofSchizosaccharomyces pombewere found to map at 16 loci,min1–min16, seven of which are allelic to knownadeloci involved in adenine biosynthesis. The mutantmin1–106was shown to be located within theade6locus by intragenic recombination analysis. Methionine sensitivity of these mutants is reversible by adenine supplementation and their pigmentation is similar to that of the correspondingademutants. We conclude, therefore, that theseminmutants are incompletely blocked in adenine biosynthesis. These observations further indicate that adenine biosynthesis inS. pombeis inhibited by methionine or a metabolite derived from it. For some otherminmutants showing reversibility of methionine sensitivity by leucine or proline, no allelism to knownleuorproloci was detected.

Spheroplast fusion and heterokaryon formation in Mucor racemosus

Heterokaryons of Mucor racemosus were produced by fusion of spheroplasts from two auxotrophic strains of the fungus. Germinated sporangiospores were converted to spheroplasts by using commercial chitinase and Myxobacter AL-1 chitosanase. Spheroplasts from the auxotrophic strains were mixed in a buffered Ca(NO3)2 solution and fusion occurred. After cell wall regeneration, prototrophs were isolated. The frequency of heterokaryon formation was 1.45 X 10(-4). Prototrophic isolates segregated parental nuclei at a high frequency, indicating that heterokaryons had formed.

Cell wall analysis of an adenine-requiring mutant of the yeast-like form of Paracoccidioides brasiliensis strain IVIC Pb9

An adenine-requiring mutant of Paracoccidioides brasiliensis strain IVIC Pb9 was isolated after treatment of the yeast-like (Y) form with nitrosoguanidine. Cell wall analysis of this mutant (strain IVIC Pb141) showed an increase in the amount of alpha-1,3-glucan and a virtual disappearance of the antigenic galactomannan. At the same time, a higher degree of virulence was observed for the mutant. These results agree with the hypothesis presented before (16) about a relationship between cell wall polysaccharides and pathogenicity in P. brasiliensis.

Nucleic acid biosynthesis in rat embryo cells infected with X14 or H-1 parvovirus

Nucleic acid biosynthesis was studied in rat embryo cell (REC) cultures 48 hours after infection with X14 or H-1 parvovirus. The incorporation of 14C-formate and [6-(14C]-orotic acid into purines and pyrimidines of various was lowered after infection with these parvoviruses. 14C-Formate incorporation into acid-soluble thymine was greatly inhibited in H-1 virus-infected cells whereas it was slightly inhibited in X14 virus-infected cells. These results suggest that X14 virus-infected cells can carry out the biosynthesis of thymidylic acid utilizing some endogenous pyrimidine nucleotide (e.g. deoxycytidylic acid, via deoxyuridylic acid). In the infected cells, the nucleoplasmic RNA polymerase activity was strongly inhibited. This results suggests an interference by the two viruses with hosts RNA synthesis.

The product of the ade1: gene in Schizosaccharomyces pombe: a bifunctional enzyme catalysing two distinct steps in purine biosynthesis

The assignment of the known ade genes to steps in purine biosynthesis in Schizosaccharomyces pombe has been completed with the demonstration that an ade3 mutants lacks FGAR amidotransferase, ade1A mutants lack GAR synthetase and ade1B mutants lack AIR synthetase. A comparison of enzyme activity with map position for ade1 mutants shows that (1) complementing ade1A mutants lack GAR synthetase but posses wild type amounts of AIR synthetase, (2) complementing ade1B mutants lack AIR synthetase but posses variable amounts of GAR synthetase, (3) non-complementing mutants lack both activities. In wild type strains the two activities fractionate together throughout a hundred-fold purification. Hence the ade1 gene appears to code for a bifunctional enzyme catalysing two distinct steps in purine biosynthesis. The two activities are catalysed by two different regions of the polypeptide chain which can be altered independently by mutation. Gel filtration studies on partially purified enzymes from wild type and various complementing mutant strains, indicate that the bifunctional enzyme is a multimer consisting of between four and six sub-units of 40,000 daltons each. GAR synthetase activity is associated with both the monomeric and multimeric forms but AIR synthetase is only associated with the multimer. A comparison of enzyme levels between diploids and their original complementing haploid strains suggests that complementation is due to hybrid enzyme formation.

Turnover at nicotinamide adenine dinucleotide in cultures of human cells

The rate of turnover of nicotinamide adenine dinucleotide (NAD) in the human cell line, D98/AH2, has been estimated by measuring the rates of entry into and exit from NAD molecules of 14C-adenine. In one set of experiments, cells were labeled by growth in medium containing 14C-adenine for six hours and then shifted to medium without labeled adenine. The loss of 14C-adenine from the adenine nucleotide and pyridine nucleotide pools was measured, and the data were analyzed using an analytical treatment which corrects for the relatively slow turnover of precursor pools. The loss of 14C-adenine from the NAD pool and from the precursor ATP pool could be related to the absolute rate of NAD breakdown. Under the experimental conditions used, the rate of NAD turnover ranged from 83,000 to 126,000 molecules per second per cell. In a complementary experiment cells were grown in the presence of unlabeled adenine, then shifted into medium containing 14C-adenine and the rate of entry of 14C-adenine into adenine and pyridine nucleotides was measured. The data were treated using a similar analysis to relate the rate of entry of 14C-adenine into NAD and the precursor ATP pools to the absolute turnover rate of NAD. This analysis gave a value for NAD turnover of 78,000 molecules per second per cell in excellent agreement with results from the pulse-chase experiments. The results from both types of experiment indicate that within D98/AH2 cells the half-life of an intact NAD molecule is 60 +/- 18 minutes. Thus, in a human D98/AH2 cell growing with a generation time of 24 hours, NAD is turning over at twice the rate found in Escherichia coli with a generation time of half an hour.

Adenine formation from adenosine by mycoplasmas: adenosine phosphorylase activity

Mammalian cells have enzymes to convert adenosine to inosine by deamination and inosine to hypoxanthine by phosphorolysis, but they do not possess the enzymes necessary to form the free base, adenine, from adenosine. Mycoplasmas grown in broth or in cell cultures can produce adenine from adenosine. This activity was detected in a variety of mycoplasmatales, and the enzyme was shown to be adenosine phosphorylase. Adenosine formation from adenine and ribose 1-phosphate, the reverse reaction of adenine formation from adenosine, was also observed with the mycoplasma enzyme. Adenosine phosphorylase is apparently common to the mycoplasmatales but it is not universal, and the organisms can be divided into three groups on the basis of their use of adenosine as substrate. Thirteen of 16 Mycoplasma, Acholeplasma, and Siroplasma species tested exhibit adenosine phosphorylase activity. M. lipophilium differed from the other mycoplasmas and shared with mammalian cells the ability to convert adenosine to inosine by deamination. M. pneumoniae and the unclassified M. sp. 70-159 showed no reaction with adenosine. Adenosine phosphorylase activity offers an additional method for the detection of mycoplasma contamination of cells. The patterns of nucleoside metabolism will provide additional characteristics for identification of mycoplasmas and also may provide new insight into the classification of mycoplasmas.

Identification of chick chromosomes in cell hybrids formed between chick erythrocytes and adenine-requiring mutants of Chinese hamster cells

When chick erythrocytes were fused with adenine-requiring mutants of Chinese hamster cells of two different complementation classes and grown in adenine-free medium for 1 week or longer, mitotic cells were observed containing both chick and Chinese hamster metaphase chromosomes within a single cell. Stable hybrid clones were subsequently isolated that possessed, in addition to a complete Chinese hamster genome, a single specific chick chromosome. Hybrids resulting from fusions involving different adenine mutants retained different, identifiable chick chromosomes. Thus, the two chick genes for the endogenous synthesis of adenine were assigned to the respective chick chromosomes.

A9145, a new adenine-containing antifungal antibiotic: fermentation

A9145 is a basic, water-soluble, antifungal antibiotic which is produced in a complex organic medium by Streptomyces griseolus. The metabolite has a molecular weight of 510, and contains adenine as well as sugar hydroxyl and amino groups. Although glucose, fructose, glucose polymers, and some long-chain fatty acid methyl esters supported biosynthesis, oils were superior, with cottonseed oil being preferred. Several ions and salts, especially Co(2+), PO(4) (3-), and CaCO(3), were stimulatory. Adenine, nucleosides, and some amino acids increased the accumulation of A9145 in shaken-flask fermentors. Enrichment of the culture medium with tyrosine afforded maximal enhancement of antibiotic production in both flask and tank fermentors. Control of the dissolved O(2) level was also critical, the optimal concentration being 3 x 10(-2) to 4.5 x 10(-2) mumole of O(2)/ml. Optimization of various fermentation parameters increased antibiotic titers approximately 135-fold in shaken flask fermentors and 225-fold in stirred vessels.

Genetics of somatic mammalian cells: demonstration of a human esterase activator gene linked to the adeB gene

Prototrophic hybrids formed from an adenine-requiring Chinese hamster cell and human fibroblasts uniformly display new esterase activity that differs from that of either parental cell in electrophoretic mobility and substrate specificity. The hybrids that grew in the selective medium and possessed the new esterase activity had a single extra chromosome that resembled a B-group human chromosome. When clones of such hybrid cells were cultured in nonselective medium, they rapidly reverted to inability to synthesize adenine, disappearance of the new esterase activity, and simultaneous loss of the extra human chromosome. Esterase activity like that of the hybrid is present in cells of various Chinese hamster, but not human, tissues. It is postulated that particular Chinese hamster esterase genes became inactive after longterm cultivation, and that, in the hybrid cell, a human activator gene linked to the adeB gene and located on a human B-group chromosome reactivated expression of these Chinese hamster esterase genes.