Biochemical characterization of a paraquat-tolerant mutant of Escherichia coli

The biochemical basis for paraquat tolerance was investigated using one of the paraquat-resistant Escherichia coli mutants previously isolated. When grown in the absence of paraquat (PQ2+), the specific activities of glucose-6-phosphate dehydrogenase and NADPH:PQ2+-diaphorase, both required for the expression of PQ2+ toxicity, were comparable in the wild type and the mutant. However, growth in the presence of 1 mM PQ2+ resulted in greater induction of these two enzymes in the wild type than in the mutant. Nevertheless, when the mutant was grown in 50 mM PQ2+, the activities of these two enzymes were comparable to those of the wild type grown in the presence of 1 mM PQ2+. Measurement of cyanide-resistant respiration, an indication of intracellular superoxide generation, showed that the intracellular flux of superoxide mediated by subsaturating concentrations of paraquat was significantly lower in the mutant than in the wild type. Extracellular superoxide formation, as measured by superoxide dismutase-inhibitable cytochrome c reduction, was higher in the wild type than in the mutant whether grown in the absence or the presence of PQ2+. The mutant did not show cross-resistance toward juglone or plumbagin, compounds known to exacerbate superoxide generation. The kinetics of [14C]PQ2+ uptake showed that the wild type accumulated PQ2+ against a concentration gradient, whereas the mutant seemed to do so only by facilitated diffusion. The results indicate that the impaired paraquat uptake system in the mutant results in the physiological and biochemical differences observed between the wild type and mutant.     

Regulation of early enzymes of ergosterol biosynthesis in Saccharomyces cerevisiae.

In order to determine the regulation mechanisms of ergosterol biosynthesis in yeast, we developed growth conditions leading to high or limiting ergosterol levels in wild type and sterol-auxotrophic mutant strains. An excess of sterol is obtained in anaerobic sterol-supplemented cultures of mutant and wild type strains. A low sterol level is obtained in aerobic growth conditions in mutant strains cultured with optimal sterol supplementation and in wild type strain deprived of pantothenic acid, as well as in anaerobic cultures without sterol supplementation. Measurements of the specific activities of acetoacetyl-CoA thiolase, HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) synthase and HMG-CoA reductase (the first three enzymes of the pathway), show that in cells deprived of ergosterol, acetoacetyl-CoA thiolase and HMG-CoA synthase are generally increased. In an excess of ergosterol, in anaerobiosis, the same enzymes are strongly decreased. A 5-10-fold decrease is observed for acetoacetyl-CoA thiolase and HMG-CoA synthase. In contrast, HMG-CoA reductase is only slightly affected by these conditions. These results show that ergosterol could regulate its own synthesis, at least partially, by repression of the first two enzymes of the pathway. Our results also show that exogenous sterols, even if strongly incorporated by auxotrophic mutant cells, cannot suppress enzyme activities in aerobic growth conditions. Measurement of specific enzyme activities in mutant cells also revealed that farnesyl pyrophosphate thwarts the enhancement of the activities of the two first enzymes.     

Coexpression of mutant and wild type protein kinase in lymphoma cells resistant to dibutyryl cyclic AMP.

A mutant clone resistant to dibutyryl cyclic AMP was isolated from S49 mouse lymphoma cells. The mutant expressed a form of cyclic AMP-dependent protein kinase distinguishable from wild type kinase by its decreased sensitivity to activation by cyclic AMP and its increased thermal lability. Hybrids formed between mutant and wild type cells were resistant to dibutyryl cyclic AMP and expressed both mutant and wild type activities in about equal amount. The parent mutant cells also appeared to express wild type kinase activity, but at a lower level. We conclude that wild type S49 cells have and express two identical alleles for the regulatory subunit of protein kinase, one of which has undergone mutation in the mutant cells.     

Mutagenesis of the folC gene encoding folylpolyglutamate synthetase-dihydrofolate synthetase in Escherichia coli

The folC gene of Escherichia coli, cloned in a pUC19 vector, was mutagenized by progressive deletions from both the 5' and the 3' ends and by TAB linker insertion. A number of 5'-deleted genes, which had the initiator ATG codon removed, produced a truncated gene product, in reduced amounts, from a secondary initiation site. The most likely position of this site at a GTG codon located 35 codons downstream of the normal start site. This product could complement the folC mutation in E. coli strain SF4 as well as a strain deleted in the folC gene. The specific activity of extracts of the mutant enzyme are 4-16% that of the wild type enzyme for the folylpolyglutamate synthetase activity and 6-19% for the dihydrofolate synthetase activity. The relative amount of protein expressed by the mutant, compared to the wild type, in maxicells was comparable to the relative specific activity, suggesting that the kcat of the mutant enzyme is similar to that of the wild type. Mutants with up to 14 amino acids deleted from the carboxy terminal could still complement the folC deletion mutant. Seven out of ten linker insertions dispersed through the coding region of the gene showed complementation of the folC mutation in strain SF4 but none of these insertion mutants were able to complement the strain containing a deleted folC gene. None of the carboxy terminal or linker insertion mutants had a specific activity greater than 0.5% that of the wild type enzyme. The dihydrofolate synthetase and folylpolyglutamate synthetase activities behaved similarly in all mutants, both retaining a large fraction of the wild type activity in the amino terminal deletions and both being very low in the carboxy terminal deletions and linker insertion mutants. These studies are consistent with a single catalytic site for the two activities catalyzed by this enzyme.     

NaCl 胁迫对小麦抗盐突变体根液泡膜H+-ATP酶 和H+-PP酶活性的影响

随着盐胁迫强度（ ＮａＣｌ ０ ～１５０ｍ ｍｏｌ／Ｌ） 和时间（０ ～７２ ｈ） 的增加,小麦抗盐突变体根液泡膜Ｈ＋ＡＴＰ 酶和Ｈ＋ＰＰ 酶活性显著增加,虽然野生型酶活性在盐胁迫下也有增加,但其增加的幅度显著低于突变体,Ｈ＋ＰＰ酶活性的差异更为显著。Ｈ＋ＡＴＰ酶和Ｈ＋ＰＰ酶的最适ｐＨ 值在两者之间以及盐胁迫前后均无改变,分别为７ ．０ 和８ ．０ 。无盐胁迫下野生型液胞膜５８ ｋＤ 蛋白带缺失,盐胁迫下这一蛋白有微弱的表达。与此不同,突变体５８ ｋＤ 蛋白在盐胁迫前后均有明显的表达,但其２９ ｋＤ 蛋白带在无盐胁迫下明显减弱。     

Cyclic AMP phosphodiesterase in Salmonella typhimurium: characteristics and physiological function.

The physiological function of cyclic AMP (cAMP) phosphodiesterase in Salmonella typhimurium was investigated with strains which were isogenic except for the cpd locus. In crude broken-cell extracts the properties of the enzyme were found to be similar to those reported for Escherichia coli. The specific activity in the mutant was less than 1% that in the wild type. Rates of cAMP production in the mutant were as much as twice those observed in the wild type. The amount of cAMP accumulated when cells grew overnight with limiting glucose was 4.5-fold greater in the mutant than in the wild type. The intracellular concentration of cAMP in the two strains was measured directly, using four different techniques to wash the cells to remove extracellular cAMP. The cAMP level in the cpd strain was only 25% greater than in the wild type. The functional concentration of the cAMP receptor protein-cAMP complex was estimated indirectly from the specific activity of beta-galactosidase in the two strains after introducing F'lac. When cells were grown with carbon sources permitting synthesis of different levels of cAMP, the specific activity of the enzyme was at most 25% greater in the cpd strain. The cpd strain was more sensitive to the effects of exogenous cAMP. Exogenous cAMP relieved both permanent and transient catabolite repression of the lac operon at lower concentrations in the cpd strain than in the wild type. When cells grew with glucose, glycerol, or ribose, exogenous cAMP inhibited growth of the mutant strain more than the wild type.     

Mechanisms of the selenium tolerance of the Arabidopsis thaliana knockout mutant of sulfate transporter SULTR1;2

We investigated the mechanism of selenium (Se) tolerance using an Arabidopsis thaliana knockout mutant of a sulfate transporter, sultr1;2. Se stress inhibited plant growth, decreased chlorophyll contents, and increased protein oxidation and lipid peroxidation in the wild type, whereas the sultr1;2 mutation mitigated damage of these forms, indicating that sultr1;2 is more tolerant of Se than the wild type is. The accumulation of symplastic Se was suppressed in sultr1;2 as compared to the wild type, and the chemical speciation of Se in the mutant was different from that in the wild type. Regardless of Se stress, the activities of ascorbate peroxidase, catalase, and peroxidase in the mutant were higher than in the wild type, while the activity of superoxide dismutase in the mutant was the same as in the wild type. These results suggest that the sultr1;2 mutation confers Se tolerance on Arabidopsis by decreasing symplastic Se and maintaining antioxidant enzyme activities.     

Effect of carbon source on enzymes involved in glycerol metabolism inNeurospora crassa

Specific activities of eight enzymes involved in glycerol metabolism were determined in crude extracts of three strains ofNeurospora crassa after growth on six different carbon sources. One of the strains was wild type, which grew poorly on glycerol as sole carbon source; the other two were mutant strains which were efficient glycerol utilizers. A possible basis for this greater effeciency of glycerol utilization was catabolite repression of glyceraldehyde kinase by glycerol in wild type, and two-fold higher glycerate kinase activity in the mutant strains after growth on glycerol, thus apparently allowing two routes for glyceraldehyde to enter the glycolytic pathway in the mutant strains but only one in wild type. The preferential entry of glyceraldehyde to the glycolytic pathway through glycerate was suggested by the lack of glyceraldehyde kinase in all three strains after growth on one or more of the carbon sources and the generally higher levels of aldehyde dehydrogenase and of glycerate kinase than of glyceraldehyde kinase.     

Inactivation of ycf33 results in an altered cyclic electron transport pathway around photosystem I in Synechocystis sp. PCC6803

ycf33 encodes a small protein with a molecular mass of 7.5 kDa and is found from cyanobacteria to higher plants. A ycf33 deletion mutant was constructed in Synechocystis sp. PCC6803 and characterized. The mutant showed a higher phycobilisome/chlorophyll ratio than the wild type and a higher photosystem II/photosystem I fluorescence ratio measured at 77 K. Under photoautotrophic conditions, the growth rates were not much different from those of the wild type. Cyclic electron transport activities around photosystem I were not much different between the wild type and the mutant. However, the effects of diphenyleneiodonium, an inhibitor of flavoprotein, on cyclic electron transport in the mutant were different from those in the wild type; it was severely inhibited in the wild type but not much in the mutant. Together with the effects of nitrite, which accepts electrons from ferredoxin via nitrite reductase and those of HgCl2, it was suggested that the pathway of cyclic electron transport is altered in the mutant.     

Mutation W284L of the human delta opioid receptor reveals agonist specific receptor conformations for G protein activation

Intrinsic activities of different delta opioid agonists were determined in a [35S]GTPgammaS binding assay using cell membranes from Chinese hamster ovary (CHO) cells stably expressing the wild type (hDOR/CHO) or W284L mutant human delta opioid receptor (W284L/CHO). Agonist binding affinities were regulated more robustly by sodium and guanine nucleotide in W284L/CHO than in hDOR/ CHO cell membranes. The W284L mutation selectively reduced the affinity of SNC 80 while having moderate effect ((-) TAN 67) or no effect (DPDPE) on the affinities of other delta selective agonists. The mutation had opposite effects on the intrinsic activities of agonists belonging to different chemical classes. The effects of the mutation on agonist affinities and potencies were independent from its effects on the intrinsic activities of the agonists. Maximal stimulation of [35S]GTPgammaS binding by SNC 80 was 2-fold higher in W284L mutant cell membranes than in wild type hDOR/CHO cell membranes, despite lower receptor expression levels in the W284L/CHO cells. The binding affinity of SNC 80 however, was significantly reduced (15-fold and 30-fold in the absence or presence of sodium+GDP respectively) in W284L/CHO cell membranes relative to wild type hDOR/CHO membranes. Conversely, the Emax of (-)TAN 67 in the [35S]GTPgammaS binding assay was markedly reduced (0.6-fold of that of the wild type) with only a slight (6-fold) reduction in its binding affinity. The affinity and intrinsic activity of DPDPE on the other hand remained unchanged at the W284L mutant hDOR. The mutation had similar effects on the affinities potencies and intrinsic activities of (-)TAN 67 and SB 219825. The results indicate that delta opioid agonists of different chemical classes use specific conformations for G protein activation.     

Control of photorespiratory glycolate metabolism in an oxygen-resistant mutant of Chlorella sorokiniana

Under a gas atmosphere of 99% O2/1% CO2, wild-type cells of Chlorella sorokiniana excreted 12% of their dry weight as glycolate during photolithotrophic growth, whereas mutant cells excreted glycolate at only 3% of the cellular dry weight. The observed difference in glycolate excretion by the two cell types appears to be due to a different capacity for the metabolism of glycolate, rather than to a different glycolate formation rate. This was concluded from experiments in which the metabolism of glycolate via the glycine-serine pathway was inhibited by the addition of isoniazid. Under such conditions, glycolate excretion rates for both cell types were identical. The mutant appeared to have significantly higher specific activities of glycine decarboxylase, serine hydroxymethyltransferase, serine-glyoxylate aminotransferase, glycerate kinase, and phosphoglycolate phosphatase than did the wild type. The specific activities of D-ribulose-1,5-bisphosphate carboxylase/oxygenase, glycolate dehydrogenase, glyoxylate-aminotransferase, and hydroxypyruvate reductase were the same for wild-type and mutant cells. The internal pool sizes of ammonia and amino acids increased in wild-type cells grown under high-oxygen concentrations but were hardly affected by high oxygen tensions in the mutant cells. Our results indicate that, under the growth conditions applied, the decarboxylation of glycine becomes the rate-limiting step of the glycine-serine pathway for the wild-type cells of C. sorokiniana.     

Altered prephenate dehydratase in phenylalanine-excreting mutants of Brevibacterium flavum.

The regulatory properties of three key enzymes in the phenylalanine biosynthetic pathway, 3-deoxy-D-arabino-heptulosonate 7-phosphate synthetase (DAHP synthetase) [EC 4.1.2.15], chorismate mutase [EC 5.4.99.5], and prephenate dehydratase [prephenate hydro-lyase (decarboxylating), EC 4.2.1.51] were compared in three phenylalanine-excreting mutants and the wild strain of Brevibacterium flavum. Regulation of DAHP synthetase by phenylalanine and tyrosine in these mutants did not change at all, but the specific activities of the mutant cell extracts increased 1.3- to 2.8-fold, as reported previously (1). Chorismate mutase activities in both the wild and the mutant strains were cumulatively inhibited by phenylalanine and tyrosine and recovered with tryptophan, while the specific activities of the mutants increased 1.3- to 2.8-fold, like those of DAHP synthetase. On the other hand, the specific activities of prephenate dehydratase in the mutant and wild strains were similar, when tyrosine was present. While prephenate dehydratase of the wild strain was inhibited by phenylalanine, tryptophan, and several phenylalanine analogues, the mutant enzymes were not inhibited at all but were activated by these effectors. Tyrosine activated the mutant enzymes much more strongly than the wild-type enzyme: in mutant 221-43, 1 mM tyrosine caused 28-fold activation. Km and the activation constant for tyrosine were slightly altered to a half and 6-fold compared with the wild-type enzyme, respectively, while the activation constants for phenylalanine and tryptophan were 500-fold higher than the respective inhibition constants of the wild-type enzyme. The molecular weight of the mutant enzyme was estimated to be 1.2 x 10(5), a half of that of the wild-type enzyme. The molecular weight of the mutant enzyme was estimated to be 1.2 X 10(5) a half of that of the wild type enzyme, while in the presence of tyrosine, phenylalanine, or tryptophan, it increased to that of the wild-type enzyme. Immediately after the mutant enzyme had been activated by tyrosine and then the tyrosine removed, it still showed about 10-fold higher specific activity than before the activation by tyrosine. However, on standing in ice the activity gradually fell to the initial level before the activation by tyrosine. Ammonium sulfate promoted the decrease of the activity. On the basis of these results, regulatory mechanisms for phenylalanine biosynthesis in vivo as well as mechanisms for the phenylalanine overproduction in the mutants are discussed.     

Mutants of Escherichia coli defective in membrane phospholipid synthesis. Properties of wild type and Km defective sn-glycerol-3-phosphate acyltransferase activities.

The sn-glycerol-3-phosphate (glycerol-P) acyltransferase, the first enzyme of membrane phospholipid synthesis in Escherichia coli, was investigated in a wild type and a mutant strain defective in this activity. The mutant strain, selected as a glycerol-P auxotroph, was previously shown to contain a glycerol-P acyltransferase activity with an apparent Km for glycerol-P 10 times higher than that of its parent or revertants. The membranous mutant glycerol-P acyltransferase but did not appear to be thermolabile in vivo. Revertants no longer requiring glycerol-P for growth, showed glycerol-P acyltransferase activity with thermolability properties similar to the wild type. The second phospholipid biosynthetic enzyme, 1-acylglycerol-P acyltransferase, was not thermolabile in membranes containing a thermolabile glycerol-P acyltransferase activity. The pH optimum for the mutant acyltransferase was over 1 pH unit higher than that of the parental activity. Further, the mutant and wild type glycerol-P acyltransferase differed in their response to magnesium chloride and potassium chloride. The palmitoyl-CoA dependence of the wild type and mutant glycerol-P acyltransferase activities were different. The mutant glycerol-P acyltransferase activity was inhibited greater than 90% by Triton X-100 under conditions where the wild type activity was not affected. These experiments provide novel information about the wild type glycerol-P acyltransferase activity of E. coli and provide six additional lines of evidence for the mutant character of the glycerol-P acyltransferase in the mutant strains.     

Kinetic characterization in batch and continuous culture of Escherichia coli mutants affected in phosphoenolpyruvate metabolism: differences in acetic acid production

The growth kinetics of an Escherichia coli wild type strain and two derivative mutants were examined in batch cultures and in glucose-limited chemostats. One mutant (PB12) had an inactive phosphotranferase transport system and the other (PB25) had interrupted pykA and pykF genes that code for the two pyruvate kinase isoenzymes. In both batch and continuous culture, important differences in acetic acid accumulation and other metabolic activities were found. Compared to the wild type strain, we observed a reduction in acetic acid accumulation of 25 and 80% in PB25 and PB12 strains respectively, in batch culture. Continuous culture experiments revealed that compared to the other two strains, PB25 accumulated less acetic acid as a function of dilution rate. In continuous cultures, oxidoreductase metabolic activities were substantially affected in the two mutant strains. These changes in turn were reflected in different levels of biomass and CO₂ production, and in oxygen consumption.     

Thermal limits and chemotaxis in mutants of the nematodeCaenorhabditis elegans defective in thermotaxis

Summary Four mutant strains of the nematodeCaenorhabditis elegans previously isolated as defective in thermotaxis (Hedgecock and Russell, 1975) were compared to the wild type in tests of their thermal range of activity and chemotaxis. The cold side of the temperature-activity curves of all four strains were different from wild type. The curves of the two cryophilic strains (EH65 and EH67) were shifted to colder temperatures. The curves of the other two mutant strains were shifted to warmer temperatures. In tests of chemotaxis to a variety of stimuli, strain EH61 made no response to any, EH71 made weak responses to all, and the remaining two strains made responses equal to wild type except for weaker responses to three chemical stimuli. It is concluded that thermotaxis shares specific gene requirements with processes controlling both thermal limits and sensory reception.     

Alterations by a defect in a rice G protein α subunit in probenazole and pathogen-induced responses

The rice dwarf1 (d1) mutant, which is deficient in an α subunit (Gα) of heterotrimeric G protein, was used to obtain specific evidence on the functions of Gα protein in defence signalling in rice. Using proteome analysis, a probenazole‐inducible protein (PBZ1) was detected in the cytosolic fraction of leaf blade of the wild type, but not the d1 mutant. After treatment with probenazol, PBZ1 reached maximal levels at 72 h in the wild type but 96 h in the d1 mutant. The induction of PBZ1 by probenazole treatment was inhibited by protein kinase inhibitors. A 48‐kDa putative mitogen‐activated protein kinase (MAPK) and a 55‐kDa putative Ca²⁺‐dependent protein kinase (CDPK) showed lower activities in the cytosolic fraction of the d1 mutant than that of the wild type. The activities of these protein kinases were enhanced at 24 h in the wild type and 48 h in the d1 mutant after probenazole treatment. Although the d1 mutant responded to the rice blast fungus similarly to the wild type, the d1 mutant developed rice blight symptoms earlier than the wild type when infected with Xoo. In addition, the blight symptoms were more severe on the mutant than on the wild type, and wilting was frequently observed in the d1 mutant. Furthermore, induction by the bacterial infection of the 48‐kDa putative MAPK and PBZ1 was delayed by 2 and 4 d, respectively, in the d1 mutant compared with the wild type. These results indicate that the Gα protein plays a role in the induction of PBZ1 and protein kinases by probenazole and Xoo, and suggest that the 48‐kDa putative MAPK may be involved in a signalling pathway for resistance to bacterial infection.     

Properties of mutationally altered RNA polymerases II of Drosophila

We tested and compared several in vitro properties of wild type and mutant RNA polymerases II from Drosophila melanogaster, using several different mutants of a single X-linked genetic locus, RpIIC4 (Greenleaf, A. L., Weeks, J. R., Voelker, R. A., Ohnishi, S., and Dickson, B. (1980) Cell 21, 785-792); the mutants tested included the original amanitin-resistant mutant, C4, which is nonconditional, plus the temperature-sensitive mutants A9, C20, E28, and 1Fb40. Using a tritium-labeled amanitin derivative, we demonstrated that C4 polymerase has a reduced binding affinity for amanitin. The C4 polymerase was as stable to thermal denaturation as the wild type enzyme, and the two enzymes had similar specific activities, ionic strength and Mn2+ requirements, and apparent Km values for UTP and GTP when assayed in the presence of Mn2+. However, with Mg2+ as the divalent cation, C4 polymerase was less active than wild type and had 2-fold higher apparent Km values for UTP and GTP. Three of the temperature-sensitive mutants, A9, C20, and E28, were derived from the amanitin-resistant mutant C4; the polymerase II activities from these mutants displayed resistance to alpha-amanitin in vitro identical with that of the C4 enzyme. C20, E28, and 1Fb40 polymerases were markedly less stable to thermal denaturation in vitro than wild type polymerase. The results presented indicate that the mutations at the RNA polymerase locus (RpIIC4-) directly alter the structure of the enzyme, providing conclusive evidence that the locus is a structural gene for a polymerase II subunit.