Optimal conditions for selecting specific auxotrophs of Saccharomyces cerevisiae using temperature-sensitive suicide mutants.

Of 48 temperature-sensitive mutants of Saccharomyces cerevisiae examined, five belonging to the same complementation group were found to undergo extensive loss of viability at the restrictive temperature. These mutants were protected from the lethal effects of exposure to a non-permissive temperature by starving for an auxotrophic requirement. By analogy with the method described by Littlewood [6] for selecting antibiotic-sensitive mutants, these temperature-sensitive mutants were found suitable in enriching for specific auxotrophs. Optimal conditions have been determined for selecting specific auxotrophs after mutagenesis by N-methyl-N'-nitro-N-nitrosoguanidine. These enable 20-fold enrichment and at least in the case of mutation to adenine dependence the method does not appear to favour mutations at any particular locus.     

Temperature-sensitive revertants

About 50% of a series of non-temperature-sensitive, spontaneously revertable auxotrophs of Escherichia coli showed temperature-sensitive revertants when the selection was at 25° rather than at 37°. This procedure provides a simple device to select temperature-sensitive mutants.     

Isolation of Spontaneously Derived Mutants of CAULOBACTER CRESCENTUS

Caulobacter crescentus has a penicillinase which precludes the use of penicillin for mutant enrichment. However, two other antibiotics, fosfomycin and D-cycloserine, can be enrich for C. crescentus mutants. In enrichment procedures for C. crescentus auxotrophs, spontaneously derived mutants occur at a frequency of 5-10% among the survivors of an enrichment procedure. Consequently, large numbers of mutants are readily obtained without any need for mutagenesis. These mutants are heterogeneous both with regard to the type of mutation and to the nutritional requirement. A similar procedure has been used to isolate temperature-sensitive mutants.     

Comparative pathogenicity of auxotrophic mutants of Candida albicans

An induced mutant of Candida albicans with greatly decreased virulence for mice is described. The mutant was one of five auxotrophic mutants obtained by ultraviolet irradiation of a clinical isolate (strain MY 1044). The five mutants included two methionine auxotrophs, one methionine-cysteine auxotroph, one temperature-sensitive serine auxotroph, and one auxotroph with unknown growth requirements. Each of the mutants produced normal mycelium and had a normal profile of susceptibility to four antifungal drugs. The virulence of each mutant was compared with the parent strain by LD50 determination in mice. Four of the five auxotrophs exhibited LD50's that were not significantly different from the parent strain (mean LD50 = 7.5 x 10(5) cells). However, the temperature-sensitive serine auxotroph was significantly less virulent than the parent strain (LD50 greater than 10(7) cells), even though it grew well in vivo and in mouse serum at 37 degrees C in vitro. Use of this mutant in conjunction with its "isogenic" parent should help to elucidate true virulence factors in C. albicans.     

The isolation and preliminary characterisation of auxotrophic and analogue resistant mutants of the moss, Physcomitrella patens

Summary Eighteen nutritional mutants have been isolated in the haploid, monoecious moss, Physcomitrella patens: five nicotinic acid auxotrophs, four p-aminobenzoic acid auxotrophs, four adenine auxotrophs, two amino acid requiring mutants and three nitrate non-utilising mutants. Seventeen of them were obtained using total isolation; one was isolated selectively. Strains resistant to the amino acid analogues, D-serine and p-fluorophenyl-alanine, and the purine analogue, 8-azaguanine, have been selected. Many of the auxotrophs are self-sterile. Crosses between auxotrophic strains have been effected and the progeny analysed. No linkage has been detected. Nicotinic acid auxotrophy has resulted from mutation in at least two genes. Self-sterility segregates as a pleiotropic effect of four mutations which produce nutritional dependence. A diploid strain has been obtained by aposporus regeneration from a hybrid sporophyte and the phenotypes of progeny resulting from the self-fertilisation of this strain have been analysed.     

Optimization on the selection of auxotrophic mutants in Candida utilis by snail enzyme treatment

The determination of the best conditions for the application of the snaill enzyme digestion method in the enrichment of auxotrophic mutants in Candida utilis was carried out following Box and Wilson's mathematical method. The selection procedure proposed was tested in the enrichment of auxotrophic mutants from a mutagenized culture of a wild-type strain. Mutant frequency was increased 46-fold by treatment with snail enzyme. The method also proved useful in the selection of additional auxotrophic mutations from single auxotrophs.     

Mutagenesis and chromosome mobilization in Hyphomicrobium facilis B-522.

Spontaneously derived antibiotic-resistant mutants of Hyphomicrobium facilis B-522, a restricted facultative methylotroph, occurred at a high frequency on agar plates with low antibiotic concentrations. Mutants specifically defective in methanol oxidation have been obtained using an allyl alcohol direct selection technique. By chemical mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine in the presence of chloramphenicol several stable auxotrophic mutants could be isolated: three leucine auxotrophs, two threonine auxotrophs, and two leucine-methionine double auxotrophic mutants. Optimal conditions for transposon mutagenesis have been developed by comparing several transposon delivery vectors. With the suicide plasmid pRK2013 as a vector, the tetracycline resistance conferring transposon Tn5-132 was introduced into the genome of H. facilis B-522. The following insertion mutants have been obtained: leu-3::Tn5-132, ilv-1::Tn5-132, and pur-1::Tn5-132. Broad host range IncP-1 plasmids could be successfully transferred by interspecific matings. Chromosome mobilization was demonstrated with the conjugative IncP-1 plasmids RP1, R68.45, pMO60, and H. facilis 2189 (leu-2, met-1, mox-1, nfs-1, str-12) as recipient strain. Transconjugants occurred at frequencies ranging from 10(-6) to 10(-8) for each marker.     

A screening procedure for the efficient recognition of Escherichia coli K-12 mutants

A method is described for the rapid screening of large numbers of E. coli colonies for detection of auxotrophs and mutants defective in sugar or succinate metabolism. The procedure permits recognition of forward mutations in a large number of functions and is applicable to the isolation of temperature-sensitive mutants in essential and nonessential genes.     

Production and study of Bacillus subtilis mutants for genes involved in nucleoside catabolism]

By means of selection for a low thymine requirement the mutants fo thymine auxotrophs for deoxyriboaldolase (dra) and phosphodeoxyribomutase (drm) genes were obtained. Besides the mutants for pyrimidinenucleoside phosphorylase gene (pdp) were olso isolated using selection on the fluorodeoxyuridine resistance. The latter enzyme provides for pyrimidine nucleosides catabolism (thymidine, uridine) in Bacilli, as well as the conversion of exogenous thymine to thymidine in thymine auxotrophs. The data obtained when studying the deo-enzymes activities in various types of the mutants and also under the condition of induction by thymidine and acetoaldehyde are in accordance with the assumption that deoxyriboso-5-phosphate is an inductor of the deo-enzymes in Bacillus subtilis. The genes dra and pdp were tightly linked as it had been shown by the transformation experiments; in contrast, no linkage was revealed between dra and drm or pdp and drm. A secondary mutation (adn), not linked with dra and blocking the ability of bacteria to catabolise adenosine (purine nucleoside phosphorylase activity remains constant) was found in some dra-mutants.     

Selective enrichment of aromatic amino acid auxotrophs in Hansenula polymorpha.

Aromatic amino acid auxotrophs of the methanol-utilizing yeast Hansenula polymorpha were effectively selected by the use of nystatin and a medium that inhibits the growth of tyrosine auxotrophs. The procedure resulted in a frequency of aromatic auxotrophs of 2% of survivors and an enrichment of 20-fold. The new procedure also takes less time than traditional procedures. Of the auxotrophic mutants isolated, two-thirds required tyrosine and the remainder were tyrosine-phenylalanine double auxotrophs.     

Selective enrichment of aromatic amino acid auxotrophs in Hansenula polymorpha.

Aromatic amino acid auxotrophs of the methanol-utilizing yeast Hansenula polymorpha were effectively selected by the use of nystatin and a medium that inhibits the growth of tyrosine auxotrophs. The procedure resulted in a frequency of aromatic auxotrophs of 2% of survivors and an enrichment of 20-fold. The new procedure also takes less time than traditional procedures. Of the auxotrophic mutants isolated, two-thirds required tyrosine and the remainder were tyrosine-phenylalanine double auxotrophs.     

The development of a negative selection system for the isolation of plant temperature-sensitive auxin auxotrophs

Summary A protocol has been developed for the negative selection of plant auxotrophs using the nucleoside analogues BUdR and FUdR. The protocol was optimised using nitrogen-starved protoplast-derived cells of Nicotiana plumbaginifolia to simulate auxotrophy. The present results represent a significant improvement over previous reports in that: 1) The background of colonies escaping BUdR/FUdR kill is low and reproducible. 2) The protocol was improved to the point where background survival was 0.03% for non-starved cultures and 0.09% for auxin-starved cultures. 3) It was shown that UV irradiation decreases BUdR sensitivity of dividing cells and that this is overcome by increased exposure to BUdR. 4) Application of the method to auxin-starved haploid protoplast-derived cell suspensions resulted, for the first time, in the selection of temperature-sensitive (ts) auxin auxotrophs. 5) It could be demonstrated, for the first time, that the method in practice enriches for auxotrophs, in this case by a factor of 10 for auxin auxotrophs and at least 60 for ts auxin auxotrophs.Abbreviations BUdR 5-bromodeoxyuridine - FUdR 5-fluoro-deoxyuridine - MNNG N-methyl-N-nitro-N-nitrosoguanidine - NAA 1-naphthaleneacetic acid - BAP 6-benzylamino-purine - CFE colony forming efficiency - PE plating efficiency - ts temperature sensitive     

The effect of nitrosoguanidine on carotene-synthesizing and pigment-free yeasts

A number of mutants with a demand for amino acids, vitamins and nitrous bases has been obtained under effect of nitrosoguanidine (0.05%) on the yeast Candida utilis and carotene-synthesizing yeast Rhodosporidium diobovatum, Rhodotorula glutinis var. glutinis and Rh. rubra. Concentration of auxotrophs due to the death of prototrophs has been achieved in the studied yeast, with the exception of Rh. rubra using additional treatment by levorin (200 units/ml). When selecting quickly growing mutants of carotene-synthesizing yeast obtained after treatment by nitrosoguanidine, the primary selection by the intensity of red-orange colour of the colonies proved to be more efficient than that by resistance to monoiodoacetic acid. The selected mutants of the pigmented yeast surpassed by primary culture as to the harvest of carotenoids (including beta-carotene) and biomass in the periodic and continuous processes.     

Mutant enrichment of Schizosaccharomyces pombe by inositol-less death.

Enrichment procedures, such as those utilizing inositol-less death, have proven to be extremely powerful for increasing the efficiency of identification of spontaneous mutants in a variety of procaryotic and eucaryotic organisms. We characterized inositol-less death in several widely used strains of the inositol-requiring yeast Schizosaccharomyces pombe and determined conditions under which this phenomenon can be used to enrich for mutants. Conflicting reports in the literature on the effects of inositol starvation upon viability of S. pombe had cast doubt on the suitability of using inositol-less death in a mutant enrichment procedure for this organism. We determined that inositol-less death was strain dependent, with differences in viability of up to 5 orders of magnitude observed between the most-sensitive strain, 972, and the least-sensitive strain, SP837. Inositol-less death was also dependent upon the cell concentration at the time of initiation of starvation. While inositol-less death occurred at all four temperatures tested, the kinetics of death was slower at 16 degrees C than at 23, 30, or 37 degrees C. Inositol-less death was observed during growth in fermentable and nonfermentable carbon sources, although loss of viability in glycerol-ethanol was significantly slower than that in glucose, sucrose, or raffinose. The feasibility of exploiting inositol-less death to enrich for spontaneous mutants was demonstrated by the identification of amino acid auxotrophs, nucleotide auxotrophs, carbon source utilization mutants, and temperature-sensitive mutants. By varying starvation conditions, some mutants were recovered at frequencies as high as 5.7 x 10(-2), orders of magnitude higher than the spontaneous mutation rate.     

Temperature-sensitive glutamate dehydrogenase mutants of Salmonella typhimurium.

Mutants of Salmonella typhimurium defective in glutamate dehydrogenase activity were isolated in parent strains lacking glutamate synthase activity by localizcd mutagenesis or by a general mutagenesis combined with a cycloserine enrichment for glutamate auxotrophs. Two mutants with temperature-sensitive phenotypes had glutamate dehydrogenase activities that were more thermolabile than that of an isogenic control strain. Eight other mutants had less than 10% of the wild-type glutamate dehydrogenase activity. All the mutations were cotransducible with a Tn10 element (zed-2:Tn10) located at approximately 23 U on the S. typhimurium linkage map. These data strongly indicate that this region contains the structural gene (gdhA) for glutamate dehydrogenase.     

转座子Tn917在短小芽孢杆菌中的转座作用

Transposition Tn917 was introduced into Bacillus pumilus 289 by protoplast transformation with plasmid pTV32. The temperature-sensitive replication property of pTV32 was maintained in B. pumilus. Tn917 was transposed efficiently in B. pumilus with 4.8 x 10(-4) transposition rate. The yield of auxotrophs was about 0.65% in all insertional mutants. It indicated a prospects for the use of Tn917 as a tool for insertional mutagenesis and genetic manipulation in B. pumilus.     

Characterization of Mutations That Suppress the Temperature-Sensitive Growth of the Hpr1Δ Mutant of Saccharomyces Cerevisiae

The hpr1Δ3 mutant of Saccharomyces cerevisiae is temperature-sensitive for growth at 37° and has a 1000-fold increase in deletion of tandem direct repeats. The hyperrecombination phenotype, measured by deletion of a leu2 direct repeat, is partially dependent on the RAD1 and RAD52 gene products, but mutations in these RAD genes do not suppress the temperature-sensitive growth phenotype. Extragenic suppressors of the temperature-sensitive growth have been isolated and characterized. The 14 soh (suppressor of hpr1) mutants recovered represent eight complementation groups, with both dominant and recessive soh alleles. Some of the soh mutants suppress hpr1 hyperrecombination and are distinct from the rad mutants that suppress hpr1 hyperrecombination. Comparisons between the SOH genes and the RAD genes are presented as well as the requirement of RAD genes for the Soh phenotypes. Double soh mutants have been analyzed and reveal three classes of interactions: epistatic suppression of hpr1 hyperrecombination, synergistic suppression of hpr1 hyperrecombination and synthetic lethality. The SOH1 gene has been cloned and sequenced. The null allele is 10-fold increased for recombination as measured by deletion of a leu2 direct repeat.     

Effect of the Osmotic Pressure of the Growth Medium on fabB Mutants of Escherichia coli

Temperature-sensitive, unsaturated fatty acid (fabB) auxotrophs of Escherichia coli can grow at the restrictive temperature in the absence of unsaturated fatty acid in a medium with a high osmotic pressure. If a mutant culture was starved for unsaturated fatty acids and harvested just before the lysis started, the fatty acid composition of the cells was the same as that of cells grown until late log phase in a high-osmotic medium. Evidence is presented that the in vivo unsaturated fatty acid biosynthesis is significantly increased in a high osmotic medium. The increase is probably due to a partial activation of the temperature-sensitive fabB product. Besides the stimulation of the temperature-sensitive fabB product, a minimal osmotic pressure of the medium appeared to be necessary to allow growth of cells containing lipids with a changed fatty acid composition. fabA mutants are unable to grow in a high-osmotic medium in the absence of unsaturated fatty acids. No increase in the in vivo unsaturated fatty acid biosynthesis could be detected in the temperature-sensitive fabA mutants.     

Mutants of Escherichia coli Defective in Membrane Phospholipid Synthesis: Mapping of sn-Glycerol 3-Phosphate Acyltransferase Km Mutants

plsB mutants of Escherichia coli are sn-glycerol 3-phosphate auxotrophs which owe their requirement to a K(m) defect in sn-glycerol 3-phosphate acyltransferase, the first enzyme in the phospholipid biosynthetic pathway. We have located the plsB gene at minute 69 of the E. coli genetic map, far removed from the gene defined by mutants with a temperature-sensitive sn-glycerol 3-phosphate acyltransferase. The plsB gene was cotransduced with the dctA locus, and the transduction data indicated that the clockwise gene order is asd, plsB, dctA, xyl. plsB(-) is recessive to plsB(+) and all acyltransferase K(m) mutants tested lie very close to the plsB locus. Effective supplementation of plsB mutants was shown not to require a defective glpD gene.