Vladislav Běhal

Mode of action of microbial bioactive metabolites.

Pathogenic microorganisms can be suppressed by cell wall destruction. Biosynthesis of peptidoglycans forming bacterial cell wall is interrupted by glycopeptides which inhibit polymerization of a disaccharide formed byN-acetylglucosamine andN-acetylmuramic acid, β-lactams and their derivatives inhibit peptidoglycan cross-linking. Antibiotics inhibiting protein synthesis bind to different sites on the rRNA and interfere with the formation of the polypeptide chain. Tumor cells resistant to chemotherapeutic drugs overproduce proteins transporting the drugs out of cells; these proteins eliminate substances which inhibit transcription of transport proteins. Some antitumor drugs (anthracyclines, fluoroquinolones, acridinesetc.) act at topoisomerases which irreversibly bind to DNA and inhibit DNA synthesis. Immunosuppressants affect various components of the immune system such as T-helper, T-effector cell function, antigen presentation and B-cell function. Antiparasitics — avermectins — bind to a receptor of this Gab-gated chlorine channel in the nerve fiber of nematodes and anthropodes, increasing the permeability of the membrane for chloride ions; the increased transport of chloride ions into the cell causes the death of the parasite. Ionophores dissolve in phospholipid bilayers and enormously increase their ionic permeability. Respiration inhibitors block the transport of electrons at several places of the respiratory chain. Rifamycin binds to the β subunit of bacterial RNA polymerase, thereby blocking mRNA synthesis. Antiviral compounds inhibit the transcription of DNA by several mechanisms or by inhibition of viral entry into host cells.

Alternative sources of biologically active substances

The majority of antibiotics and substances with diverse biological activity used in medicine are produced by actinomycetes, nonfilamentous bacteria and fungi. Other microorganisms, such as myxobacteria, pseudomonads, nocardias, basidiomycetes, marine microorganisms, enterobacteria, halobacteria, hyperthermophilesetc. are investigated for new biologically active metabolites.

Purification and characterization of a novel valine dehydrogenase from Streptomyces aureofaciens

The first valine dehydrogenase of S. aureofaciens had been described (Vancurová, I., Vancura, A., Volc, J., Neuzil, J., Flieger, M., Basarová, G. and Bĕhal, V. (1988) J. Bacteriol. 170, 5192-5196). In the present work, a second valine dehydrogenase was detected and purified by hydrophobic and fast protein liquid chromatographies. The enzyme has a relative molecular mass (M(r)) of 240,000 and is composed of 6 identical subunits, each of M(r) 41,000. In the presence of NAD, the enzyme catalyzes the reversible deamination of several branched- and straight-chain amino acids. The enzyme activities with L-2-aminobutyrate and deamino-NAD+ are markedly higher than those with L-valine and NAD+, respectively. The enzyme synthesis is significantly induced by L-valine but severely repressed by ammonia. Molecular and catalytic properties of the enzyme distinguish it from the other described valine dehydrogenases. The results directly demonstrate the presence of two valine dehydrogenases in a single Streptomyces species.

What type of glutamine synthetase is important forStreptomyces coelicolor A3(2) under nitrogen-limited growth conditions?

SummaryGlutamine synthetase I activity ofStreptomyces coelicolor was strongly repressed by ammonia and was induced 56.8 fold in a nitrogen-free medium. Glutamine synthetase II activity was not induced even by a long-term nitrogen starvation. Therefore, glutamine synthetase I is the only active enzyme ofStreptomyces coelicolor.

The induction of valine dehydrogenase activity from Streptomyces by L-valine is not repressed by ammonium

SummaryActivity of valine dehydrogenases (VDH) from Streptomyces aureofaciens and S. fradiae is strongly induced by L-valine even in the presence of 25mM NH4+. When added into 16 h-old cultures growing with 100mM NH4+, L-valine induced the synthesis of VDH. The results indicate that Streptomyces can utilize L-valine in the presence of NH4+, and the induction of VDH activity by L-valine is not repressed by NH4+.

DoesStreptomyces aureofaciens contain two alanine dehydrogenase isozymes

SummaryTwo NAD-dependent dehydrogenases exhibiting activity with L-alanine were detected in crude extracts ofStreptomyces aureofaciens. The dominant enzyme was found to be an alanine dehydrogenase with 6 or 4 subunits, each ofM, 45,000. The minor enzyme exhibited high activities with branched-chain amino acids, suggesting it is a valine dehydrogenase. The results demonstrate thatS. aureofaciens has only one alanine dehydrogenase, which is catalytically active in a six- or four-subunit structure.

How is glutamine synthetase I activity from Streptomyces aureofaciens regulated

SummaryActivity of glutamine synthetase I (GSI) from Streptomyces aureofaciens increased markedly during tetracycline production phase. The purified GSI exhibited a low affinity for glutamate but high affinities for ATP and ammonium. Its Mn2+-dependent activity was more sensitive to feedback inhibitors than Mg2+-dependent activity. Both the activities were significantly stimulated by Co2+ but inhibited by other divalent cations. ADP was a strong inhibitor. These results suggest that GSI activity is regulated by availability of substrates, feedback inhibitors, divalent cations, and cell energy charge.

Identification and characterization of a heat-labile type I glutamine synthetase fromStreptomyces cinnamonensis

Streptomycetes have two distinct glutamine synthetases (GS): a heat-stable dodecameric GSI and a heat-labile octameric GSII. A heat-inactivated GS activity was detected in crude extracts ofStreptomyces cinnamonensis cells grown with nitrate or glutamate as the nitrogen source. The purified enzyme obtained from crude extracts of the nitrate-grown cells after affinity and anion-exchange chromatography was also heat-labile; it was inactivated by 80 % when incubated at 50 °C for 1 h. However, the enzyme has properties typical of GSI and similar with those of the heat-stable GSI purified fromS. aureofaciens: It is composed of twelve subunits, each ofM 55 kDa, and has a native molar mass of 625 kDa and an isoelectric point at pH 4.2. In addition, its activity is regulated by reversible adenylylation. Mg2+ and NaCl but not Mn2+ protected the purified enzyme from thermal inactivation, and both NaCl and Mn2+ or Mg2+ stabilized its activity at 4–8 °C. As compared with GSI fromS. aureofaciens, theS. cinnamonensis enzyme was cleaved more extensively during SDS-PAGE, was less sensitive to feedback inhibitors, and similarly affected by divalent cations. TheKm values were 12.5 mmol/L forl-glutamate, 0.1 for NH4+, 1.25 for ATP, 18.5 forl-glutamine, 3.3 for hydroxylamine and 0.087 for ADP. To our best knowledge, this is the first report of a heatlabile GSI from any source.

Removal of Mn2+ induces dissociation of glutamine synthetase fromStreptomyces aureofaciens

Removal of Mn2+ by EDTA treatment converted dodecameric glutamine synthetase (GS) fromStreptomyces aureofaciens into inactive subunits but did not affect significantly their conformation. However, when fractionated by gel filtration FPLC, the Mn2+-free subunits showed a 7-fold increase ofA280, probably due to a significant alteration in their tertiary structure. Mn2+ reduced theA280 of the subunits and promoted their reaggregation to form active GS. Mg2+ or Ca2+ but not Co2+ or Zn2+ might have similar effects. The results suggest that specific divalent cations might play a crucial role in stabilizing subunit interactions as well as the conformation of the individual subunits inStreptomyces GS. The role of specific divalent cations in the regulation of GS turnover is discussed.

Regulation, purification and partial characterization of glutamine synthetase fromStreptomyces aureofaciens

The activity of glutamine synthetase (GS) fromStreptomyces aureofaciens was regulated by the availability of the nitrogen source. Rich nitrogen sources repressed GS synthesis and increased GS adenylylation. The enzyme was purified 270-fold to virtual homogeneity with 37% recovery. The molar mass of the native enzyme and its subunits was determined to be 620 and 55 kDa, respectively, indicating that GS is composed of 12 identical subunits. The enzyme has a hexagonal-bilayered structure as observed by electron microscopy. The isoelectric point of the purified GS was at pH 4.2. The enzyme was stable for 1 h at 50°C but lost activity rapidly when incubated at 65 and 70°C. Mg2+ supported relative synthetic activity of 100 and 72%, respectively, with the corresponding pH optima of 7.3 and 7.0. Mn2+ ions activated transferase activity at a pH optimum of 7.0. The temperature optimum for all GS activities was 50°C. Intermediates of the citric acid cycle exerted insignificant effects on the synthetic activities. There was no SH-group essential for the GS activity.