THE EFFECT OF 5-FLUORODEOXYURIDINE ON GENETIC REPLICATION AND MITOTIC CROSSING OVER IN SYNCHRONIZED CULTURES OF USTILAGO MAYDIS

Chlorhexidine digluconate induces mitotic recombination in diploid cells of Aspergillus nidulans

Chlorhexidine digluconate (1,1'-hexamethylene-bis[(5-p-clorophenyl)-biguanide]) is a bisbiguanidine antiseptic, used to decrease plaque formation and to control periodontal diseases. The determination of the frequency of mitotic crossing-over constitutes a very important method for detecting carcinogenic agents.

OBJECTIVE

The recombinogenic potential of chlorhexidine digluconate was evaluated on Aspergillus nidulans by the production of cells homozygous for the following nutritional markers: riboA1, pabaA124, biA1, methA17 and pyroA4.

METHOD

A. nidulans was exposed to three concentrations of chlorhexidine digluconate (1, 5, and 10 microM).

RESULTS

Inhibition of colony development, conidiophore morphological alteration (cytotoxic effect), and the recombinogenic effect, indicated by homozygotization index (HI) values higher than 2.0, were observed for all concentrations of chlorhexidine digluconate. A homozygous pyro+//pyro+ diploid strain and a diploid homozygous for the recessive w gene were isolated from UT448//A757 diploid treated with chlorhexidine digluconate, emphasazing its recombinogenic potential.

CONCLUSION

Although, beneficial effects of chlorhexidine, as an antiseptic agent, are reported in the literature, our results revealed that chlorhexidine digluconate, at less levels lowered those used clinically, caused toxic and recombinogenic effects on diploid A. nidulans strain.

Parameiosis inAspergillus nidulans in response to doxorubicin

The recombinagenic effect of doxorubicin (an anticancer agent that impairs DNA synthesis and causes chromosome breaks) was used to induce parameiotic events in Aspergillus nidulans. Heterokaryons formed with master strains and uvs mutants were inoculated with and without doxorubicin. Haploid segregants (parameiotics and parents) and aneuploids were selected as heterokaryon-derived visible sectors. Among parameiotic segregants, recombinants by intergenic mitotic crossing-over and recombinants by chromosome-independent segregation were found. Whereas segregants of the former type were obtained only with doxorubicin, those of the latter type were recovered both with and without the drug.

Early studies on recombination and DNA repair in Ustilago maydis

This historical review covers the period 1960 to mid-1980s. The first experiments were carried out at the John Innes Institute, Bayfordbury, Hertford, with a one year interlude in the Department of Genetics, University of Washington, Seattle. In 1965, I moved to the National Institute for Medical Research, Mill Hill, London, and became head of a new Genetics Division. The research on Ustilago was divided broadly into (1) experimental genetic studies, and (2) DNA enzymology, largely under the direction of the late Geoffrey Banks. The approaches involved isolating and characterizing mutants defective in repair and recombination (the first in any eukaryotic organism), with the longer term aim of identifying the function of genes through studies of enzymes and proteins which interact with DNA. An enzyme capable of recognizing mismatched bases in DNA was identified. A novel method exploited the inducible nitrate reductase gene, and revealed relationships between recombination, mutation, repair, transcription and cell survival. Several different studies provide strong evidence for the presence of an inducible repair pathway, dependent on recombination. Much more recently, the revolution in molecular genetics has been in exploited in several laboratories working with Ustilago maydis, and these have produced some completely new insights into recombination and repair.

Avaliação do potencial recombinogênico do antibiótico danofloxacina em células diplóides de Apergillus nidulans

Estudou-se o potencial recombinogênico da danofloxacina, novo antimicrobiano pertencente ao grupo das 4-fluoroquinolonas e de uso exclusivo em medicina veterinária, no fungo filamentoso Aspergillus nidulans. A linhagem mestra UT196 e o mutante Z1 foram utilizados para formar o diplóide Z1//UT196. Conídios desse diplóide foram inoculados em placas de Petri contendo meio mínimo suplementado com 2,5, 5,0 e 10,0 mig/ml de danofloxacina. As placas foram incubadas por cinco dias a 37 ºC. Segregantes mitóticos foram isolados das colônias tratadas com o antimicrobiano e as análises de seus fenótipos evidenciaram o efeito recombinogênico da danofloxacin na dose de 10,0 mig/ml. Recombinantes para vários intervalos dos cromossomos I e II foram identificados entre os segregantes analisados.

The REC1 gene of Ustilago maydis, which encodes a 3'-->5' exonuclease, couples DNA repair and completion of DNA synthesis to a mitotic checkpoint

Mutation in the REC1 gene of Ustilago maydis results in extreme sensitivity to killing by ultraviolet light. The lethality of the rec1-1 mutant was found to be partially suppressed if irradiated cells were held artificially in G2-phase by addition of a microtubule inhibitor. This mutant was also found to be sensitive to killing when DNA synthesis was inhibited by external means through addition of hydroxyurea or by genetic control in a temperature-sensitive mutant strain defective in DNA synthesis. Flow cytometric analysis of exponentially growing cultures indicated that wild-type cells accumulated in G2 after UV irradiation, while rec1-1 cells appeared to exit from G2 and accumulate in G1/S. Analysis of mRNA levels in synchronized cells indicated that the REC1 gene is periodically expressed with the cell cycle and reaches maximal levels at G1/S. The results are interpreted to mean that a G2-M checkpoint is disabled in the rec1-1 mutant. It is proposed that the REC1 gene product functions in a surveillance system operating during S-phase and G2 to find and repair stretches of DNA with compromised integrity and to communicate with the cell cycle apparatus.

Inducible phenotypic multidrug resistance in the fungus Mucor racemosus

The dimorphic fungus Mucor racemosus exhibited a single-step, inducible resistance to cycloheximide, trichodermin, and amphotericin B. Cells adapted to inhibitory levels of the antibiotics after 12 to 40 h. The adaptation involved all the cells in the population and was not the result of the selection of resistant mutants. Adaptation to one drug provided cross resistance to other, dissimilar drugs. Resistance was lost within several generations of growth in the absence of the inhibitors.

Effect of mutagens, chemotherapeutic agents and defects in DNA repair genes on recombination in F' partial diploid Escherichia coli

The ability of mutagenic agents, nonmutagenic substances and defects in DNA repair to alter the genotype of F' partial diploid (F30) Escherichia coli was determined. The frequency of auxotrophic mutants and histidine requiring (His-) haploid colonies was increased by mutagen treatment but Hfr colonies were not detected in F30 E. coli even with specific selection techniques. Genotype changes due to nonreciprocal recombination were determined by measuring the frequency of His- homogenotes, eg. F' hisC780, hisI+/hisC780, hisI+, arising from a His+ heterogenote, F' hisC780 hisI+/hisC+, his1903. At least 75% of the recombinants were homozygous for histidine alleles which were present on the F' plasmid (exogenote) of the parental hetergenote rather than for histidine alleles on the chromosome. Mutagens, chemotherapeutic agents which histidine alleles on the chromosome. Mutagens, chemotherapeutic agents which block DNA synthesis and a defective DNA polymerase I gene, polA1, were found to increase the frequency of nonreciprocal recombination. A defect in the ability to excise thymine dimers, uvrC34, did not increase spontaneous nonreciprocal recombination. However, UV irradiation but not methyl methanesulfonate (MMS) induced greater recombination in this excision-repair defective mutant than in DNA-repair-proficient strains. Mutagenic agents, with the exception of ethyl methanesulfonate (EMS), induced greater increases in recombination than the chemotherapeutic agents or the polA1 mutation. EMS, which causes relatively little degradation of DNA, was more mutagenic but less recombinogenic than MMS, a homologous compound ths that inhibition of DNA occurring single-stranded regions in replicative intermediates of the DNA. Mutagens which cause the rapid breakdown of DNA may, in addition, introduce lesions into the genome that increase the number of single-stranded regions thus inducing even higher frequencies of recombination.

Induced intragenic recombination in yeast can occur during the G1 mitotic phase

The conditional cell division cycle yeast mutants cdc have been used to demonstrate that intragenic recombination induced by ultraviolet or gamma rays occurs in diploids arrested in G1, a short time after irradiation and before the initiation of the S phase. This implies that pairing of homologous chromosomes does not require duplicated chromatids.

Increased spontaneous mitotic segregation in MMS-sensitive mutants of Saccharomyces cerevisiae

Methyl methanesulfonate (MMS)-sensitive mutants of Saccharomyces cerevisiae belonging to four different complementation groups, when homozygous, increase the rate of spontaneous mitotic segregation to canavanine resistance from heterozygous sensitive (canr/+) diploids by 13-to 170-fold. The mms8-1 mutant is MMS and X-ray sensitive and increases the rate of spontaneous mitotic segregation 170-fold. The mms9-1 and mms13-1 mutants are sensitive to X rays and UV, respectively, in addition of MMS, and increase the rate of spontaneous mitotic segregation by 13-fold and 85-fold, respectively. The mutant mms21-1 is sensitive to MMS, X rays and UV and increases the rate of spontaneous mitotic segregation 23-fold.

Resistance and mitotic instability to chloroneb and 1,4-oxathiin in Aspergillus nidulans

Mutants resistant to two fungicides, chloroneb (1,4-dichloro-2,5-dimethoxybenzene) and vitavax (2,3-dihydro-5-carboxanilido-6-methyl-1,4-oxathiin) were spontaneously obtained from a strain of Aspergillus nidulans with frequencies of 12.5 and 1.1 respectively, in 10(8) conidia. One chloroneb-resistant mutant (Chl 1) segregated as a single gene and was mapped in linkage group IV. It also caused a partial dependence of the strain on the fungicide and was semi-dominant. The mutant resistant to vitavax (Vit 1) also segregated as a single gene and was dominant. Both fungicides altered the instability of diploid and duplication strains. Chloroneb mainly increased haploidization, and vitavax reduced the mitotic recombination in diploids. Chloroneb increased the instability of duplication strains, and vitavax reduced such instability. The possible mode of action of such fungicides affecting stability is discussed.

Evidence for a new kind of regulatory gene controlling expression of genes for morphogenesis during the cell cycle in Ustilago violacea

The first part of the paper provides strong supportive evidence for the previous findings (Cummins & Day, 1973; Day & Cummins, 1973) that the two alleles of the mating-type locus of the basidiomyceteUstilago violaceahave different periods of inducibility during a cell cycle, and that the cell cycle characteristics of each allele are maintained in freshly isolated diploids. This difference in temporal properties of the alleles appears to be the basis of the dominance of allele a2as it is inducible during a phase of the cell cycle when allele a1is non-inducible. During G1both alleles appear to be inducible and apparently ‘neutralize’ each other so that the cell cannot mate.

The second part of the paper provides evidence for a unique genetic control mechanism. The evidence suggests that the period of cell cycle inducibility of a locus governing a morphogenetic pathway may be regulated by a separate control gene thecclocus, with two known allelesccstr(a stringent or restricted period of inducibility) andccrel(a relaxed or non-restricted period of inducibility). This hypothesis stems from analysis of a diploid that wasa1·ccstr/a2·ccreland showed dominance of allelea2during the S and G2phases when freshly isolated, but which became incapable of mating after a period of subculturing. Analysis of haploids derived from this diploid strain showed that both mating-type alleles were functional but that it was now homozygous forccstr, i.e. of genotypea1·ccstr/a2·ccstr· Thus the temporal and functional aspects of the mating type alleles are determined by different loci. It is postulated that cell cycle control loci may be widespread and serve to regulate the action of genes concerned with morphogenesis in relation to other cell cycle events.

Radiation-sensitive pyrimidine auxotrophs of Ustilago maydis. I. Isolation and characterization of mutants

The relationship between UV sensitivity and pyrimidine auxotrophy has been examined. Fourteen pyrimidine-requiring mutants have been classified on the basis of genetic complementation and utilization of biosynthetic intermediates and have been assigned to at least four loci. All the mutants studied were sensitive to UV, although the degree of sensitivity varied both between loci and amongst alleles at the same locus. A double mutant strain carrying pyrimidine mutants at two loci was only as sensitive to UV as the more sensitive of the singles. This suggests that both mutants are deficient in the same repair mechanism. Suppressor mutations which restored endogenous pyrimidine biosynthesis were isolated. These suppressors restored UV resistance in the presence of the original mutation. The results indicate that the UV sensitivity of these mutants is a direct result of pyrimidine auxotrophy and not to any secondary properties of the mutants.

Influence of edeine on intergenic and interallelic recombination in Neurospora crassa

The effect of edeine and the mutation edr-2 to edeine resistance on genetic recombination in Neurospora crassa was investigated. For this purpose crosses between pairs of edeine sensitive and edeine resistant strains respectively were set up without or in the presence of the drug (0-750 microg/ml). The genetic markers ylo-1, ad-1, pan-2 (B3 and B5) and tryp-2, all on linkage group VI, were used for scoring recombinants. These were ad+, tryp+ (intergenic recombination) and pan+ (interallelic recombination). Frequencies of about 6-7% for intergenic and of about 0.4% for interallelic recombination were found in crosses between eds strains and ed(r) strains respectively, if edeine was absent. However, crosses in the presence of edeine gave higher frequencies of both intergenic and interallelic recombination (about 12% intergenic and 1% interallelic with 180 to 200 microg ed/ml). The pan+ prototrophs (interallelic recombinants) obtained in the different crosses were tested for distribution of outside markers. The data thus obtained revealed that under the effect of both the mutation to edeine resistance and edeine itself the relative number of noncrossover (gene conversion) recombinants decreases in favour of crossover recombinants, and the relative number of double crossover recombinants (events outside the pan locus) decreases in favour of single crossover recombinants. It is concluded that a) edeine and the mutation ed(r)-2 to edeine resistance affect recombination via related pathways, and b) noncrossover and crossover recombinants are caused by different molecular mechanisms, in agreement with the work of other authors.

Recombination during blocked chromosome replication in temperature-sensitive strains ofUstilago maydis

tsd1–1 strains ofUstilago maydisare blocked in DNA synthesis under restrictive conditions. On recovery from the block, both intergenic and allelic recombination are enhanced, and aneuploids are generated. UV is synergistic in the induction of recombination. Direct assays of allelic recombination have shown that gene conversion occurs during the block in DNA synthesis.

Effect of putative deoxyribonucleic acid inhibitors on macromolecular synthesis in Saccharomyces cerevisiae

The effects of inhibitors of bacterial deoxyribonucleic acid (DNA) synthesis upon logarithmically growing cultures of Saccharomyces cerevisiae were investigated. Cell division, ribonucleic acid (RNA) synthesis, and DNA synthesis were measured after addition of nalidixic acid, fluorodeoxyuridine, or phenethyl alcohol to cultures of yeast growing in defined and complex media. Both nalidixic acid and fluorodeoxyuridine had only temporary effects on nucleic acid synthesis in cultures growing in defined medium, and little or no observable effect on cultures growing in complex medium. Neither compound inhibited colony formation on complex solid medium, although growth was slow on defined solid medium. Phenethyl alcohol caused complete inhibition of DNA synthesis, RNA synthesis, and cell division in cultures growing in defined medium. In cultures growing in complex medium, RNA synthesis and cell division were inhibited to a lesser extent. A slight increase in DNA was observed in the presence of the inhibitor.

A search for temperature-sensitive mutants of Ustilago maydis blocked in DNA synthesis

Over 400 temperature-sensitive mutants ofUstilago maydishave been tested for DNA synthesis at the restrictive temperature of 32°C by measuring14C adenine incorporation into DNA and RNA. Five mutants were defective in DNA synthesis but none was completely blocked. One mutant,tsd-1, which is unlinked to the others, forms long uninucleate filaments at 32°C which die exponentially after 4h temperature treatment. The phenotype is comparable to that of thymine-starved bacteria, but it is possible that rather than being specifically defective in DNA synthesis the mutant is blocked in nuclear division.

Studies on the expression of somatic crossing over in Glycine max L

Variety T219 of Glycine max L. has spontaneous yellow, dark green and double (yellow-dark green) spots on the leaves of plants of genotype Y 11 y 11 but no such spots are found on leaves of Y 11 Y 11 or y11y11 plants. It was suggested (Vig and Paddock, 1968) that the double spots result from somatic crossing over whereas the two types of single spots primarily originate from chromosomal disturbances.Cold shocks disproportionately increased the frequency of double spots, but ethyl methanesulfonate (EMS) did not do so. However, in most cases of each treatment, the frequency of single spots increased. It is suggested that EMS is not very potent in bringing about somatic recombination whereas cold shocks are. Plants from a sample of seeds of variety L65-1237 that had been harvested in 1968 at Urbana, Illinois, did not express the spotting phenomenon, but plants from seeds harvested in 1969 at Reno did have spots. Application of mitomycin C(MC) to the seeds of this variety as well as of T219 increased the incidence of double spots manyfold indicating that MC can reveal the potential for somatic crossing over in a variety which might not otherwise express it. Soaking dry seeds of L65-1237 in aqueous solutions of MC for intervals as short as 2 hours was found effective in increasing the frequency of double spots. The role of MC in relation to DNA synthesis and somatic crossing over is discussed. Application of the chromosome-breaking agent, claunomycin (DM) was ineffective in causing double spots.

INDUCED MITOTIC CROSSING-OVER IN RELATION TO GENETIC REPLICATION IN SYNCHRONOUSLY DIVIDING CELLS OF USTILAGO MAYDIS

A method is now available for synchronizing the division of populations of cells of the smut fungusUstilago maydis. In two experiments carried out with a diploid strain heterozygous for several biochemical markers, samples of cells were removed at intervals through the synchronized division cycle and treated with a constant dose of ultra-violet light. Cell survival and the frequency of the various recombinants resulting from induced mitotic crossing-over were recorded. In addition the period of DNA synthesis in the unirradiated population has been measured. During this period the cells are very sensitive to ultra-violet light and those that survive contain the highest proportion of induced recombinants. In so far as the markers make it possible to locate the position of cross-overs, cells which are irradiated early in the period of genetic replication show most crossing-over towards the ends of the chromosome arms, whilst cells which are treated late in this period show crossing-over near the centromeres. The data are most easily interpreted by supposing that chromosome replication begins at the ends of the arms and proceeds to the centromere, and that the temporary interruption of this process by ultra-violet light can result in pairing and crossing-over in the vicinity of the points of interruption.