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M L Bekker

Publications and source records attributed to M L Bekker.

At least 19 recordsLinked to original sources
PubMed1989

[Synthesis of heat-shock proteins in Saccharomyces cerevisiae protoplasts].

The effect of cellular capsule elimination in Saccharomyces cerevisiae yeasts (protoplast formation) on the heat-shock protein synthesis and the synthesis of the proteins in protoplasts were studied. The methods of mono- and dimeric electrophoresis have demonstrated that (1) about 18 heat-shock proteins with the molecular masses 26-98 Kd are synthesized in cells at 41 degrees C; (2) protoplast formation per se does not induce the synthesis of heat-shock proteins, but the induction of these proteins in protoplasts at 41 degrees C is similar to the one in intact cells. The protoplast formation induces the synthesis of specific proteins different from heat-shock proteins and the synthesis is inhibited by the heat-shock. The heat-shock induces modification of 88 and 86 Kd heat-shock proteins. It inhibits the synthesis of a number of peptides (15-50 Kd) in cells and protoplasts.

Electrophoresis, Polyacrylamide Gel↗
PubMed1986

[Apurinic-apyrimidinic DNA-endonuclease activity of cytochrome c and pancreatic RNAse].

Cationic proteins--cytochrome c and pancreatic RNAase--possess the apurinic-apyrimidinic DNA-endonuclease activity. The affinity of these proteins for DNA-apurinic sites does not differ from that of specific apurinic DNA-endonucleases described in literature. The main features of the apurinic activity of cationic proteins are as follows: low specific activity, high temperature optimum of the reaction, absence of primer-stimulated activity. The feasibility of participation of cationic proteins and some other nucleophilic compounds in single-stranded breaks production in apurinic DNA is discussed.

Animals↗
PubMed1985

[Hybrid plasmid with bacterial and fungal markers carrying the denV gene of T4 phage and restoring the UV-resistance of E. coli uvrA].

The hybrid plasmid pYBP2 with bacterial (ampR), yeast (LEU2) and bacteriophage T4 (denV) genes has been constructed. The plasmid transformed Escherichia coli CSR603 uvrA recA ampS leuA phr- to ampicillin resistance, leucine independence, UV-resistance similar to the one of uvrA+ recA strain. Cell-free extracts of transformed Escherichia coli cells contain low level of ultraviolet-endonuclease activity in contrast to nontransformed cells containing no enzyme.

Endodeoxyribonucleases↗
PubMed1984

Quantitative characterization of pyrimidine dimer excision from UV-irradiated DNA (excision capacity) by cell-free extracts of the yeast Saccharomyces cerevisiae.

Cell-free extracts from wild-type yeast (RAD+) and from rad mutants belonging to the RAD3 epistatic group (rad1-1, rad2-1, rad3-1, rad4 -1) contain activities catalyzing the excision of pyrimidine dimers (PD) from purified ultraviolet-irradiated DNA which was not pre-treated with exogenous UV-endonuclease. The level of these activities in cell-free extracts from rad mutants did not differ from that in wild-type extract and was close to the in vivo excision capacity of the latter calculated from the LD37 (about 10(4) PD per haploid genome).

DNA, Fungal↗
PubMed1982

Purification and properties of two endonucleases specific for apurinic/apyrimidinic sites in DNA from Saccharomyces cerevisiae.

Two distinct endonucleases from Saccharomyces cerevisiae, specific for apurinic/apyrimidinic sites (AP-endonucleases A and B), have been extensively purified and characterized. Both are free from unspecific and ultraviolet-specific endonucleases and exonucleases. The two enzymes are monomeric proteins of around 24000 daltons. Both are sensitive to ionic strength and most active in the presence of 150 and 100 mM NaCl for AP-endonucleases A and B, respectively. They are not absolutely dependent on divalent cations, since they are insensitive to EDTA, although AP-endonuclease A is activated by Ca2+ or Mg2+ and AP-endonuclease B by Mg2+ only. ATP inhibits the enzymes. AP-endonuclease A reacts optimally between pH 6 and 8, and AP-endonucleases B at pH 8. AP-endonuclease A is more stable at 60 degree C (half-life of 17 min) than B (half-life of 4 min). AP-endonuclease A is insensitive to N-ethylmaleimide or rho-chloromercuribenzoate. AP-endonuclease B is also insensitive to N-ethylmaleimide, but rho-chloromercuribenzoate inhibits its activity.

Adenosine Triphosphate↗
PubMed1982

[Properties of 5-phosphoryl-1-pyrophosphate amidotransferase from the yeast Saccharomyces cerevisiae wild type and mutant with altered purine biosynthesis regulation].

The properties of partially purified 5-phosphoribosyl-1-pyrophosphate amidotransferase (EC 2.4.2.14) from S. cerevisiae 15V-P4 wild type and mutant aza 165 were studied. The latter is characterized by a higher sensitively of de novo purine synthesis to the inhibitory effect of exogenous guanine. Both enzymes were stable to short-term heating at 60 degrees. The rate of the enzyme-catalyzed reaction was dependent on the enzyme and substrate concentrations. Both enzymes had identical affinities for the substrates (Km for phosphoribosyl pyrophosphate was 0.44 mM for the wild type and 0.5 mM for mutant amidotransferase; Km for glutamine was 2.6 mM for both strains). No differences in the enzyme sensitivity to AMP and IMP inhibition were observed. There were essential differences in the sensitivity to inhibition by GMP: the level of inhibition was more than 80% for mutant amidotransferase and only 35% for the wild type enzyme. The inhibition with respect to phosphoribosylpyrophosphate for the former enzyme was of a mixed type, that for the wild type enzyme was of the "non-competitive" type. This feature of amidotransferase regulation in S. cerevisiae is the cause of the decreased sensitivity of de novo purine nucleotide biosynthesis to inhibition by exogenous guanine.

Amidophosphoribosyltransferase↗
PubMed1981

Purification and properties of deoxyribonucleic acid polymerase from Bacillus stearothermophilus.

Deoxyribonucleic acid polymerase I was purified from Bacillus stearothermophilus to 50 to 70% homogeneity. Its molecular weight was 76,000. The enzyme was insensitive to sulfhydryl blocking agents and showed maximal activity at 60 degrees C, pH 8 to 9, 0.25 M KCl, and 0.02 M MgSO4. The rate of heat inactivation of the deoxyribonucleic acid polymerase followed first-order kinetics with a half-life of 90 min at 60 degrees C; the addition of 0.05% bovine serum albumin protected the enzyme, which could be heated for 180 min without loss of activity. The ratios of polymerase to nuclease activities were about 20 for 5'-3' exonuclease and more than 500 for 3'-5' exonuclease. The Km for deoxyribonucleoside-5'-triphosphates was 7 microM.

Cations↗
PubMed1980

A new mutant of the yeast Saccharomyces cerevisiae defective in excision of UV-damaged sites in DNA.

An UV-sensitive yeast mutant, uvs12, with almost unchanged sensitivity to gamma-irradiation and methylmethane sulphonate was obtained. uvs12, non allelic to any of the known UV-sensitive mutants from radI to rad21 is defective in early steps of excision repair. This inference is based on the fact that after 4-5 h post-irradiation incubation unexcised pyrimidine dimers are retained in nuclear DNA, which follows from two independent tests: the retention of UV-endonuclease-sensitive sites and enhanced survival after photoreactivation.

DNA Repair↗
PubMed1978

[Regulation of purine nucleotide biosynthesis in mutant Saccharomyces cerevisiae yeasts with increased sensitivity of the pathway for de novo synthesis to inhibition by exogenous guanine].

Aza 165 and aza 238 Saccharomyces cerevisiae mutants characterized by a 2.5 times higher sensitivity of the de novo purine synthesis to the inhibitory effect of exogenous guanine, as compared with the wild type strain, have been selected by their sensitivity to 8-azaguanine. The exogenous guanine somewhat inhibits the growth and synthesis of nucleis acids in mutants, this being due in vivo neither to permeability changes of the cell membrane, nor to concentration changes of guanilic derivatives in the acid-soluble pool of yeast cells. Using cell-free extract of the strain aza 165, it has been shown that the synthesis of the first product of metabolic pathway for de novo formation of purines, phosphoribosylamine, is inhibited by GMP by 81% and only by 35% in the 15V-P4 strain of the wild type. The inhibition by other end products, IMP and AMP, is the same in both wild and mutant strains. The enhanced sensitivity of the purine synthesis to guanine in vivo is thus due to changes in regulatory properties of the key enzyme of purine nucleotide formation, phosphoribosylpyrophosphate amido-transferase (EC 2.4.2.14). This change in the regulation of purine synthesis in yeast is likely to be a mechanism to compensate the genetically controlled defect in end steps of the biosynthesis pathway, i.e. the incapability of converting guanilic derivatives to adenilic ones. However, the information concerning the regulation of PRPP-amido-transferase activity responsible for differential sensitivity to adenilic and guanilic nucleotides in yeast is not lost but only strongly repressed.

Adenosine Monophosphate↗
PubMed1976

Utilization of exogenous pyrimidines as a source of nitrogen by cells of the yeast Rhodotorula glutinis.

Uptake and intracellular transformation of pyrimidines supplying cells of the yeast Rhodotorula glutinis with nitrogen have been studied. The amine nitrogen of cytosine was found to be the easiest to utilize. The presence in the medium of inorganic ammonia along with cytosine had a slight effect on cytosine deaminase (EC 3.5.4.1) activity. The uracil produced entered into the nutrient medium with no fission break of the pyridmidine ring. In the absence of any other source of nitrogen, the cells of the yeast R. glutinis utilized nitrogen of the pyrimidine ring of oxypyrimidines. Catabolism of uracil followed the reductive pattern, with release of carbon dioxide; this was accompanied by synthesis of the key enzyme of pyrimidine catabolism, dihydrouracil dehydrogenase (EC 1.3.1.1), whose activity rose 10-fold. With thymidne as the sole source of nitrogen, the lag-phase growth of the yeast cells was maximum. Catabolism of the pyrimidine ring of thymine was possibly preceded by its transformation into uracil. With no source of nitrogen easily utilized, the uridine 5'-monophosphate content in the generally acid-soluble pool rose. Our discussion of the regulation of catabolism of exogenous pyrimidine bases by the yeast R. glutinis takes into account the fact that transformations of pyrimidine bases are determined by how easily the cells can use a particular base as a source of nitrogen.

Aminohydrolases↗