Studies on a Nuclease from Ustilago maydis

Root Border Cells and Their Role in Plant Defense

Root border cells separate from plant root tips and disperse into the soil environment. In most species, each root tip can produce thousands of metabolically active cells daily, with specialized patterns of gene expression. Their function has been an enduring mystery. Recent studies suggest that border cells operate in a manner similar to mammalian neutrophils: Both cell types export a complex of extracellular DNA (exDNA) and antimicrobial proteins that neutralize threats by trapping pathogens and thereby preventing invasion of host tissues. Extracellular DNases (exDNases) of pathogens promote virulence and systemic spread of the microbes. In plants, adding DNase I to root tips eliminates border cell extracellular traps and abolishes root tip resistance to infection. Mutation of genes encoding exDNase activity in plant-pathogenic bacteria (Ralstonia solanacearum) and fungi (Cochliobolus heterostrophus) results in reduced virulence. The study of exDNase activities in plant pathogens may yield new targets for disease control.

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.

Sodium phosphate enhances plasmid DNA expression in vivo

Intramuscular injection of plasmid DNA results in myofiber cell expression of proteins encoded by the DNA. The preferred vehicle for plasmid DNA injections has been saline (154 mM sodium chloride) or PBS (154 mM NaCl plus 10 mM sodium phosphate). Here, it is shown that injection of luciferase or beta-galactosidase encoding plasmid DNA in a 150 mM sodium phosphate vehicle into murine muscle resulted in a two- to seven-fold increase in transgene expression compared with DNA injected in saline or PBS. When the DNA encoded secreted alkaline phosphatase, preproinsulin or interferon, sodium phosphate vehicle increased their serum levels by two- to four-fold. When the DNA encoded mouse erythropoietin, sodium phosphate vehicle increased hematocrits by two-fold compared with DNA injected in saline. When the DNA encoded influenza nucleoprotein, sodium phosphate increased anti-nucleoprotein antibody titers by two-fold. The expression of luciferase from plasmid DNA instilled into lung was increased five-fold compared with that in vehicle without sodium phosphate. Incubation of plasmid DNA with muscle extract or serum showed that sodium phosphate protected the DNA from degradation. Thus, a change from sodium chloride to sodium phosphate vehicle can enhance the expression of plasmid DNA in a tissue, possibly by inhibiting DNA degradation. Gene Therapy (2000) 7, 1171-1182.

The effect of the Saccharomyces cerevisiae endo-exonuclease NUD1 gene expression on the resistance of HeLa cells to DNA-damaging agents

HeLa cells transiently transfected with a mammalian expression DNA vector expressing the Saccharomyces cerevisiae endo-exonuclease (EE) NUD1 gene have exhibited changes in cell survival frequencies after treatment with different DNA-damaging agents as compared to HeLa cells transfected with a control plasmid. The NUD1-transfected cells showed a dose-dependent increase in sensitivity to UV irradiation resulting in up to 58% decrease in cell survival. In response to gamma-irradiation NUD1 transfected cells featured an increased survival at doses equal to and greater than 2.0 Gy, reaching a maximum enhancement in survival frequency of 17%. At the same time, the NUD1-transfectants featured an increase in resistance to 0.25 microM-0.5 microM cis-platin (up to 58% increase in cell survival) and 1.0 mM EMS (11% increase). At higher concentrations of EMS NUD1 expression resulted in a decreased cell survival of the transfected cells (17% decrease for 2.5 mM EMS). No difference in cell survival frequencies between the NUD1-transfectants and the controls was observed after treatment with different concentrations of chlorambucil and mechlorethamine. These results suggest possible roles played by EEs in different DNA repair pathways--being stimulatory for the repair of certain types of DNA lesions, such as double strand breaks (DSBs), and interfering with the endogenous DNA repair systems for the repair of other types of lesions. Furthermore, these results also provide additional indirect evidence for the role of EEs in homologous recombination.

A new meiotic endonuclease from Coprinus meiocytes

Two different types of Coprinus meiotic nuclease have been previously reported by the authors which are believed to be involved in meiotic chromosome recombination [1,2]. A third meiotic endonuclease was purified from the cap tissues of the basidiocarp of Coprinus cinereus. The enzyme is a 60 kDa molecule composed of a monopolypeptide as revealed by SDS-PAGE and FPLC-Sephacryl S-300 gel filtration. The enzyme belongs to a type of endonuclease which can preferentially digest single-stranded DNA and requires divalent cations as a co-factor, most commonly Mg2+ ions. In the presence of this co-factor, the enzyme converts the supercoiled plasmid DNA (form I) to both the relaxed form (form II) and the linear form (form III). Ca2+ ions can also function as a co-factor, though, in this case, not only is form I plasmid converted to form II, but a few ladder bands between form I and form II are also produced. The Ca2+ ion effect as a cofactor can be prevented with ATP. Immunohistochemical observation shows that the enzyme is distributed in the surface of the gills, which contain the meiotic tissues. These characteristics clearly differ from those of the meiotic nucleases reported previously.

Single-strand-specific nucleases

Single-strand-specific nucleases, which act on single-stranded nucleic acids and single-stranded regions in double-stranded nucleic acids, are multifunctional enzymes and are ubiquitous in distribution. They find wide application as analytical tools in molecular biology research, although enzymes such as P1 nuclease are also used for production of flavor enhancers such as 5' IMP and 5' GMP. Because these enzymes are mainly used as analytical tools, very little attention was paid to aspects relating to their structure-function relationships. However, during the last few years considerable developments have taken place in this area. Single-strand-specific nucleases, their purification, characteristics, biological role, and applications have been reviewed.

A Euglena gracilis zinc endonuclease

A 26-kDa endonuclease has been purified to homogeneity from zinc-sufficient Euglena gracilis. The protein binds to single-stranded DNA with a higher affinity than to double-stranded DNA, but it exhibits nucleolytic activity toward both. Thus, it converts supercoiled plasmid pBR322 DNA into the linear form, a property characteristic of endonucleases, and it continues to act on the linearized DNA until it is completely degraded. It also hydrolyzes heat-denatured, single-stranded calf thymus DNA. Moreover, at amounts below 1 microgram, it enhances RNA synthesis by RNA polymerase II, a characteristic observed with other DNases. Its addition to an in vitro transcription assay increases RNA synthesis up to 3-fold. The nuclease requires two metal components to carry out its enzymatic activities. It hydrolyzes DNA only in the presence of millimolar amounts of magnesium or micromolar quantities of other activating metal ions, such as manganese, zinc, or cobalt. However, even when optimal concentrations of Mg2+ are present, micromolar amounts of the metal-chelating agents OP and HQSA completely inhibit pBR322 digestion. Transcription enhancement is also inhibited completely by both chelators at concentrations that do not affect the intrinsic polymerase II activity. By atomic absorption spectrometry, the enzyme contains 1 g-atom of Zn/mol, which is the likely target of chelator action. The nuclease protein can also be isolated from zinc-deficient E. gracilis, but remarkably it then contains 1 mol of Cu/g-atom and no zinc.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and properties of nuclease SP

Single-strand-specific nucleases are a diverse and important group of enzymes that are able to cleave a variety of DNA structures present in duplex molecules. Nuclease SP, an enzyme from spinach, has been purified to apparent homogeneity, allowing for the unambiguous characterization of a number of its physical properties as well as its DNA strand cleavage specificities. The effects of ionic strength, pH, divalent metal cations, and temperature on nuclease SP activity have been examined in detail. Nuclease SP was found to be quite thermostable and could be stimulated by Co2+. In addition, the cleavage of UV-damaged and undamaged supercoiled plasmid substrates under a variety of conditions suggests that at least two types of structures are recognized and processed by nuclease SP: UV photoproduct-induced distortions and unwound "nuclease hypersensitive sites". These studies indicate that nuclease SP is functionally related to other single-strand-specific nucleases and is a potential enzymatic tool for probing and manipulating various types of DNA structures.

Purification and characterization of an endonuclease from fruiting caps of basidiomycete Coprinus cinereus

An endonuclease was purified from the cap tissues of basidiocarp of Coprinus cinereus collected at early meiotic prophase. It has an optimal activity at pH 7.0 and 37 degrees C. It is a cationic enzyme with a molecular mass of 22 kDa by gel filtration, and contains a 12-kDa and a 14-kDa peptide as revealed by SDS gel electrophoresis and Western blot analysis. An antiserum was produced in rabbits against the purified Coprinus endonuclease. The specificity of this antiserum was demonstrated in a dot-blot analysis and, more critically, in an immunoinhibition of endonuclease activity. The Coprinus endonuclease requires Mg2+ and/or Ca2+ as co-factors. Ca2+ is more efficient than Mg2+ while the effect of combining both co-factors is the highest. The Coprinus endonuclease has a substrate preference for single-strand and supercoiled DNA. It gives only single-strand nicks on supercoiled DNA at low enzyme concentration and limited time of incubation. At high enzyme concentration and/or long incubation time, double-strand fragmentation occurred. As is discussed, this endonuclease is believed to be involved in the early phase of meiotic recombination.

An endo-exonuclease activity of yeast that requires a functional RAD52 gene

Extracts of Rad+ and radiation-sensitive (rad) mutants of the yeast Saccharomyces cerevisiae were examined for total Mg2+-dependent alkaline deoxyribonuclease activity and the presence of a nuclease that crossreacts immunologically with an antiserum raised against an endo-exonuclease from Neurospora crassa, an enzyme exhibiting both deoxyribo- and ribonuclease activities. No significant differences were observed in total deoxyribonuclease activity between Rad+ and rad mutants. The antibody precipitable activity, however, was found to be 30%-40% of the total alkaline deoxyribonuclease activity in logarithmically growing Rad+ cells. Extracts of stationary phase cells were lacking in antibody precipitable activity. Using immunoblot methods, a 72 kDa crossreacting protein was identified from logarithmically growing cells that was absent from stationary phase cells. In all radiation-sensitive mutants examined, except rad52, at least 20% of total activity was precipitable. Extracts from logarithmically growing rad52 mutants, including a rad52::LEU2 insertion mutant, exhibited less than 10% of the Rad+ precipitable activity; however, some crossreacting material was detected. Although, the level of endo-exonuclease activity is influenced by the RAD52 gene, it is not the product of this gene. The total deoxyribonuclease and the antibody precipitable endo-exonuclease activities were also followed during meiosis. Unlike the Rad+ strain which had previously been shown to have increased levels of total and immunoprecipitable endo-exonuclease as cells underwent meiosis, the rad52 mutant exhibited no increases in either category of nuclease activity. Given the importance of the RAD52 gene in repair, recombination and mutagenesis, the endo-exonuclease may be a significant component of these processes.

Single-strand-specific nuclease of pea seeds: glycoprotein nature and associated nucleotidase activity

A single-strand-specific nuclease from germinating pea seeds has been purified to homogeneity. The purification procedure includes affinity chromatography on concanavalin A-Sepharose and gel filtration. The nuclease exhibits its activity at neutral pH and does not have an absolute requirement for a divalent cation. The purified nuclease also possesses a 3'-nucleotidase activity and is a glycoprotein containing about 20% carbohydrate. On native polyacrylamide gels the nuclease activity comigrates with the nucleotidase. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed the presence of two subunits in the native enzyme. The nuclease and nucleotidase activities show differential rates of thermal inactivation, the latter following simple first order kinetics and the former exhibiting a more complex reaction. The nucleotidase was also found to be stimulated by DNA, the increase being greater with native DNA than with denatured DNA. These properties are possibly accounted for by the dimeric structure of the enzyme where the nucleotidase catalytic site resides in one subunit while the nuclease site is formed by interaction of both subunits. The enzyme also hydrolyzes double-stranded alkylated DNA and depurinated DNA at a higher rate than native DNA. Experimental evidence suggests that depurinated DNA is hydrolyzed in the region of apurinic sites.

Single-strand-specific nuclease from rat liver endoplasmic reticulum: characterization and mode of action

1. The characteristics and mode of action of a single-strand-specific nuclease isolated from rat liver endoplasmic reticulum are investigated with respect to its DNA and RNA substrates. 2. The RNase activity of the enzyme is slightly influenced by the presence of divalent cations but the DNase activity is enhanced by divalent cations particularly Mn2+. 3. Activity is partially inhibited by the presence of EGTA; this effect is reversed most efficiently by the addition of Mn2+. 4. The enzyme exhibits small pH dependence between pH 6-9 and maximum activity is observed at pH 7-7.5 for both DNase and RNase activities. 5. Sulfhydryl group reagents do not affect its action but histidyl group reagents exert a small but definite effect. 6. The enzyme degrades DNA and RNA endonucleolytically producing fragments which possess 3'-OH and 5'-phosphate termini. 7. Monomers are not produced even after prolonged degradation. 8. The end product of poly(U)degradation ranges between two and four building blocks but the DNA product is longer probably due to considerable percentage of secondary structure.

Effects of Neurospora nuclease halo (nuh) mutants on secretion of two phosphate-repressible alkaline deoxyribonucleases

Various recently isolated nuh mutants of Neurospora crassa (i.e., mutants which show reduced nuclease haloes on DNA-sorbose plates flooded with HCl) were mapped in several new genes or gene clusters and checked for effects on DNA repair and nuclease secretion. Some of them were found to be sensitive to MMS (methylmethane sulfonate) and sterile in meiosis. Release of nuclease activities into filtrates of liquid cultures was analyzed by DEAE-Sepharose chromatography. In the wild type, three alkaline deoxyribonuclease activities (A, B, and C) can be separated after growth in sorbose minimal media [Fraser, M. J. (1979). Nucleic Acids Res. 6: 231]. When strains were grown in phosphate-free DNA sucrose media, high (200-fold derepressed) DNase levels were found, and crude dialyzed filtrates could be chromatographed. Only two peaks were found, namely, those of DNase A, a Ca2+-dependent strand-nonspecific endonuclease, and DNase B, a ss-DNA-specific Mg2+-dependent exonuclease. Of the nuh mutants analyzed by one or both of these methods, many resembled the wild type. A few showed poor derepression, since their sorbose filtrates were normal, while profiles from DNA media lacked all peaks. These grew variably in liquid media with organic phosphates and probably produced suppressors, as was regularly found for nuc-2. Other mutants, which lacked specific peaks, gave the same results with both methods. One of these, nuh-7, produced no peaks at all but secreted unusually high amounts of protein.

Biochemical genetics of Neurospora nuclease II: Mutagen sensitivity and other characteristics of the nuclease mutants

Five newnucmutants ofNeurospora crassawere characterized for their relative sensitivities to different mutagens (UV, MNG, MMS), to mitomycin-C and to histidine; latter has been shown to inhibit the growth of certain UV sensitive mutants. These mutants were also compared for their capabilities for spontaneous mutation as determined by resistance to p-fluoro-phenylalanine. Based on these characterization, the mutants seem to belong to two groups. The first group includednuc-3andnuc-6which showed sensitivity to all mutagen tested and possessed capability for a very high frequency of spontaneous mutation (i.e. mutator effect). The second group includednuc-4, nuc-5andnuc-7; these were as resistant to different mutagens as the wild type strain, but possessed an antimutator effect (i.e. the frequency of spontaneous mutation by these three mutants were at least 0·5–100 × less than the wild type strains). There was some variation in these properties of mutants belonging to the two groups. Among all the fivenucmutants,nuc-3was characterized by extreme sensitivity to all mutagens. None of the fivenucmutants were sensitive to histidine. The properties ofnucmutants are discussed in relation to their possible role in DNA repair and recombination.

Homologous pairing can occur before DNA strand separation in general genetic recombination

In the presence of ATP and Mg2+, purified Escherichia coli recA protein promotes the formation of joint molecules between closed circular duplex DNA and homologous circular single-stranded DNA carrying a short annealed fragment. The presence of this fragment is essential for pairing between molecules. In similar conditions recA protein is unable to act as a helicase and does not cause strand separation of the fragment from the single-stranded circle. Thus, homologous pairing between DNA molecules can take place without prior unwinding of a free end.

Mutagen sensitivities and mutator effects of MMS-sensitive mutants in Neurospora

7 mus (mutagen-sensitive) mutants of Neurospora crassa, which are more sensitive to the toxic effects of MMS (methyl methanesulfonate) than wild-type, were investigated for cross-sensitivities to other mutagens and inhibitors. These mutants have recently been mapped in 5 new genes, mus-7 to mus-11, and mutant alleles from each gene were checked for their effects on mutation frequencies. It was found that mutants in 3 of these 5 genes showed radiation-induced mutation frequencies similar to wild-type. These included 2 alleles of the gene mus-10, which were cross-sensitive only to UV and were the only mutants that produced some viable ascospores in homozygous crosses. The mutant of the second gene, mus-8, was especially sensitive to UV and mitomycin C and produced slightly reduced frequencies of spontaneous mutation. In contrast, the mutant of the third gene, mus-7, was not UV-sensitive but showed some cross-sensitivity to X-rays; mus-7 was highly sensitive to MMS and also to histidine, which inhibits various repair-defective mutants at concentrations well below those that reduce wild-type growth. None of these mus resemble mutants previously found in Neurospora, nor do they conform clearly to mutant types identified in E. coli or yeast. On the other hand mutants in 2 further genes, mus-11, and especially 2 alleles of mus-9, are very similar to uvs-3 of Neurospora and generally resemble mutants that are considered to be defective in "error-prone" repair. They were UV- as well as X-ray-sensitive, and showed strong spontaneous mutator effects but almost no increase in recessive lethal frequencies in heterokaryons after UV-treatments.

Nucleases and their control in wild-type and nuh mutants of Neurospora

A review of all of the work on Neurospora nucleases strongly suggests that five nucleases, originally isolated on the basis of markedly different properties, may actually be derived from a single inactive precursor polypeptide via different routes of proteolysis. One of these nucleases may be involved in DNA repair and/or recombination. Two repair-deficient mutants of Neurospora, uvs-3 and nuh-4, may have a lesion in protease(s) which control the level of this nuclease or in some function which regulates the protease(s). Both of these mutants map in the same gene region and they may be defective in recombination, since they are sensitive to various mutagens and to mitomycin C and they show high frequency of spontaneous, but not radiation-induced, recessive lethal mutations and/or deletions.

Isolation and genetic analysis of nuclease halo (nuh) mutants of Neurospora crassa

Nuclease halo (nuh) mutants of the ascomycete Neurospora crassa have been isolated which are characterized reduced release of deoxyribonuclease (DNase) activities from colonies grown on sorbose-containing agar media. To identify nuh mutants, mutagenized isolates were transferred to commercial DNase test agar, or grown on minimal medium and then overlayed with agar that contained heat-denatured DNA. DNase activity was visualized by acid precipitation which produced clear rings of digestion (haloes) around the colonies. To identify the number of genes in which mutations lead to reduced release of nuclease activity, eleven nuh mutants were checked for close linkage and linked pairs were tested for complementation. These mutants were assigned to eight genes, and all except one were mapped in six small regions of the Neurospora linkage maps. In addition, among a large number of existing mutants which were tested for nuclease haloes, two mutants were found that showed the Nuh phenotype, namely uvs-3 and uvs-6. One of the isolated nuh mutants was also found to be sensitive to UV and was mapped close to uvs-3; it may represent a new allele of this gene. As a first step towards identification of genuine nuclease mutants, extensively backcrossed strains of mutants from different genes have been assayed for nuclease activity with denatured DNA in extracts. A pronounced reduction, compared to wild type at the same stage of growth, was found in uvs-3 and also in nuh-3, a mutant that is not UV-sensitive.

A genetic and developmental analysis of an acid deoxyribonuclease in Drosophila melanogaster

A deoxyribonuclease, called D Nase-1, that is active at acid pH in the presence of EDTA has been studied in Drosophila melanogaster. The locus for the Enzyme maps genetically to 61.8 on the right arm of the third chromosome. Cytogenetically, DNase-1 has been localized to within five to ten bands between 90C-2 and 90E. This analysis utilizes both electrophoretic variants and the Y-autosome translocations of Lindsley et al. (1972). DNase-1 is present in all stages of the life cycle, and the paternal genome actively contributes DNase-1 to the ambryo between 0 and 1 hr after fertilization.

The recognition of mismatched base pairs in DNA by DNase I from Ustilago maydis

The activity of Ustilago maydis DNase I, an enzyme implicated in genetic recombination, on DNA substrates containing unpaired or mismatched bases, was examined. The enzyme nicked supercoiled PM-2 molecules, converting these to relaxed circular and linear molecules. Discrete double stranded linear fragments smaller than unit length were also observed after digestion at high enzyme concentration. Heteroduplex molecules were constructed using phi80 bacteriophage derivatives which contained single base substitutions within the E. coli tRNA1tyr gene. Single and double stranded nicking at or near the single mismatched site was observed with three out of the five pairs of heteroduplexes.

Biochemical analysis of genetic recombination in eukaryotes

Recent studies concerning molecular mechanisms of genetic recombination in eukaryotes are reviewed. Since many of these studies have focused on the testable predictions arising from the hybrid DNA theory of genetic recombination, this theory is summarised. Experiments to determine the time of meiotic crossing-over and the structure of the synaptonemal complex which facilitates meiotic crossing-over are described. Investigations of DNA nicking and repair events implicated in recombination are discussed. Properties of proteins which may facilitate hybrid DNA formation, and biochemical evidence for hybrid DNA formation are presented. Finally, a nuclease which has been implicated in gene conversion is described.

A deoxyribonuclease from Chlamydomonas reinhardii. 2. Substrate specificity, mode of action and products

A deoxyribonuclease purified Chlamydomonas reinhardii has been shown to be specific for single-stranded DNA. The enzyme is most active on thermally denatured DNA, but also degrades single-stranded termini from double-stranded DNA. The enzyme has no effect on single-stranded or double-stranded intact circular phiX174DNA, suggesting that it requires DNA termini for activity. DNA is digested progressively to oligonucleotides and then mononucleotides. The product of the reaction is nucleoside 5'-monophosphates. The enzyme has no effect on RNA, nor does it possess phosphatase or phosphodiesterase activity. No specificity was demonstrated for phosphate or hydroxyl groups at either the 5' or 3' termini of DNA. The enzyme may be able to initiate hydrolysis at either the 3' or the 5' termini, since radioactivity was released more rapidly from 5' and 3' termini than from bulk DNA. The enzyme has been tentatively named Chlamydomonas reinhardii exonuclease 1.

The use of a novel plate assay in a search for yeast mutants defective in deoxyribonucleases

Methods by which the intracellular enzymes deoxyribonuclease, ribonuclease and protease can be assayed in whole colonies of Saccharomyces cerevisiae on agar plates are described. A search for mutants deficient in deoxyribonuclease has been carried out. Two types of mutant are described. One apparently fails to produce deoxyribonuclease, ribonuclease or protease on agar plates and the other apparently fails to produce deoxyribonuclease and ribonuclease.

Differential chromosomal and mitochondrial DNA synthesis in temperature-sensitive mutants of Ustilago maydis

The amount and type of residual DNA synthesis was determined in eight temperature-sensitive mutants of the smut fungus Ustilago maydis after incubation at the restrictive temperature (32 degrees C) for eight hours. Mutants ts-220, ts-207, ts-432 and ts-346 were found to have an overall reduction in the synthesis of both nuclear and mitochondrial DNA comparison to the wild-type. In mutants ts-20, tsd 1-1, ts-84 and pol 1-1 nuclear DNA synthesis was depressed relative to mitochondrial synthesis. The DNA-polymerase mutant pol 1-1 had persistent nuclear synthesis at about 50% of the rate of synthesis of mitochondrial DNA and similar behavior was observed in a diploid homozygous strain. Mutant ts-84 had an initial burst of DNA synthesis which was reduced for nuclear but not mitochondrial synthesis after three hours preincubation at 32 degrees C. tsd 1-1 and ts-20 had nuclear residual synthesis amounting to about 25% of the relative rate of mitochondrial synthesis which correlates to increasing UV sensitivity of these strains on incubation at 32 degrees C. A pol 1-1 ts-84 double mutant had an additive loss of nuclear DNA synthesis which indicates that the steps of replication involved may be sequential.

An endonuclease from mouse cells specific for single-stranded DNA

An endonuclease with a molecular weight of about 70000 (5-6S) was extensively purified from mouse ascites cells. The enzyme specifically attacks single-stranded DNA which is degraded mainly to oligonucleotides, with 5-10 residues. Supercoiled covalently closed circular phage DNA is converted to the linear relaxed form. The enzyme activity is highly sensitive to salt and can be stimulated by reagents lowering the dielectric constant of the buffer such as dimethylsulfoxide and glycerol.

A novel endonuclease of human cells specific for single-stranded DNA

We have fractionated from human aneuploid cell cultures three different enzyme fractions degrading single-stranded DNA. We have purified and characterized one of these DNases; this is an endonuclease working at alkaline pH (around 9.5) and requiring Mg2+ for its activity. The enzyme degrades denatured DNA over 100 times more efficiently than native DNA in optimal conditions. The termini produced by the enzyme have 5'P and 3'OH ends. The enzyme can attack, though at reduced rate, the supertwisted circular molecule of Simian virus 40 DNA, whereas it is inactive on the nicked circular molecule. The ultraviolet irradiation of DNA, whether native or denatured, does not affect its efficiency as substrate of the DNase. The properties of this endonuclease distinguish it from those of the other DNases described previously in mammalian cells; the denomination DNase VI is therefore proposed. Its properties are similar to those of DNases described in Ustilago maydis and Bacillus subtilis, for which an essential role in recombination seems likely.

Evidence for the formation of hybrid DNA during mitotic recombination in Chinese hamster cells

Direct evidence is provided for the formation of hybrid DNA during mitotic recombination in CHO cells. The cells were labeled for one round of replication in medium containing BUdR, so that the density of the DNA was heavy light (HL) and then returned to light medium. Further DNA synthesis, during either repair or chromosome replication, can only result in HL or fully light (LL) DNA; however, the formation of hybrid DNA as part of the process of recombinational repair will produce some fully heavy (HH) DNA. A small fraction of DNA containing regions of HH DNA has been detected on neutral CsC1 gradients, and the amount of this DNA is increased by treatment of the cells with mitomycin C. Increasing doses of mitomycin C produce smimlar increases in both the amount of HH DNA and the frequency of sister chromatid exchanges measured cytologically. This correlation provides evidence that the HH DNA is hybrid DNA, formed as an intermediate in recombinational repair.

Genetic characterization of rec-1, a mutant of Ustilago maydis defective in repair and recombination

Detailed physiological and genetic studies of haploid and diploid strains have revealed a complex phenotype for therec-1 mutation inUstilago maydis. The mutant is defective in the repair of damage by UV light, ionizing radiation and nitrosoguanidine. Four alleles are all recessive and have the same sensitivity to UV, suggesting the loss of a single cellular function. A significant fraction of non-viable cells is formed during growth, and in diploid strains considerable variation in colony size and morphology is seen. The spontaneous frequency of mutation is greater than in wild-type cells, but there is little, if any, enhancement by irradiation.

rec-1 also has pleiotropic effects on genetic recombination. The spontaneous level of mitotic allelic or non-allelic recombination is abnormally high, but the relative increase after irradiation is much lower than in control diploids. Allelic recombination is strongly associated with the expression of a hetozygous recessive distal marker, and it is shown that this is often due to hemizygosity rather than to homozygosity of this marker. The results indicate that allelic recombination is due to crossing over rather than gene conversion, but that the cross over is often associated with a chromatid break.rec-1 interacts with other radiation sensitive mutants, such asrec-2. Diploids homozygous for both are totally deficient in allelic recombination. In crosses betweenrec-1 strains meiosis is defective, with a low viability of meiotic products and frequent production of aneuploids or diploids among the survivors. The overall phenotype ofrec-1 strains can best be explained in terms of the loss of a regulatory function, which leads to uncontrolled recombination during mitosis and meiosis, and the loss of a recombination repair pathway which is normally induced by agents which damage DNA.

Enzymes of Neurospora crassa which attack UV-irradiated DNA

Two excision-deficient mutants of Neurospora crassa contain normal levels of two enzymes, a single-strand-specific exonuclease and a single-strand specific endonuclease, which attack UV-irradiated DNA. Mutants of N. crassa have been obtained in which the activity of the latter enzyme as well as an activity with native DNA are simultaneously affected. These activities are also apparently low in excision-proficient uvs3 strain of N. crassa which has many of the characteristics of the recA mutants of E. coli.