Purification and characterization of actinomycin synthetase I, a 4-methyl-3-hydroxyanthranilic acid-AMP ligase from Streptomyces chrysomallus.

Actinomycin synthetase I was purified to homogeneiety from actinomycin-producing Streptomyces chrysomallus. The purified enzyme is a single polypeptide chain of M(r) 45,000. It catalyzes the formation of the adenylate of 4-methyl-3-hydroxyanthranilic acid (4-MHA) from the free acid and ATP in an equilibrium reaction. 4-MHA is the precursor of the chromophoric part of actinomycin. By using the 4-MHA analogue, 4-methyl-3-hydroxybenzoic acid, as a model substrate it could be established that the equilibrium constant Keq is independent on enzyme concentration, which suggests that no stoichiometric acyladenylate-enzyme complex is formed in contrast to observations made with aminoacyl adenylates formed by aminoacyl-tRNA synthetases or multifunctional peptide synthetases. Actinomycin synthetase I does not charge itself with substrate carboxylic acid via a covalent thioester bond as is usual for amino acid activation in non-ribosomal peptide synthesis. In addition, the enzyme does not act as an acyl-coenzyme A ligase as revealed by its inability to release AMP in the presence of 4-MHA or other structurally related aromatic carboxylic acids, coenzyme A and ATP. Additional analysis of the activation reaction showed that it is exothermic, whereas the free enthalpy change delta G0 is positive due to a negative entropy change indicating a strong influence of restriction of random motion on the course of the reaction. Determinations of Km and kcat of various substrate carboxylic acids revealed the highest kcat/Km ratio for the natural substrate 4-MHA. From these properties, actinomycin synthetase I represents the prototype of novel chromophore activating enzymes involved in non-ribosomal synthesis of chromopeptide lactones in streptomycetes.     

Aromatic C-methyltransferases with antipodal stereoselectivity for structurally diverse phenolic amino acids catalyze the methylation step in the biosynthesis of the actinomycin chromophore

The actinomycin biosynthetic gene cluster of Streptomyces chrysomallus harbors two paralogous genes, acmI and acmL, encoding methyltransferases. To unveil their suspected role in the formation of 3-hydroxy-4-methyl-anthranilic acid (4-MHA), the building block of the actinomycin chromophore, each gene was expressed in Escherichia coli. Testing the resulting ～40 kDa His(6)-tagged proteins with compounds of biogenetic relevance as substrates and S-adenosyl-l-methionine revealed that each exclusively methylated 3-hydroxykynurenine (3-HK) with formation of 3-hydroxy-4-methylkynurenine (4-MHK) identified by its in vitro conversion to 4-MHA with hydroxykynureninase. AcmI and AcmL methylate also hydroxyphenyl-amino propanoic acids such as p-tyrosine, m-tyrosine, or 3,4-dihydroxy-l-phenylalanine (DOPA) but at a lower rate than 3-HK. The presence of the α-amino group was necessary for substrate recognition. Phenolic acids with shorter chains such as 4-hydoxyphenyl-l-glycine (HPG), 3-hydroxybenzoic acid (3-HB), or 3-hydroxyanthranilic acid (3-HA) gave no product. Both enzymes were stereospecific for the optical configuration at α-C with unprecedented antipodal selectivity for the d-enantiomer of 3-HK and the l-enantiomer of p-tyrosine or m-tyrosine. AcmI and AcmL show sequence similarity to various C- and O-methyltransferases from bacteria. Phylogenetic analysis places them into the clade of C-methyltransferases comprising among others orthologues involved in 4-MHA formation of other biosynthesis systems and methyltransferases putatively involved in the C-methylation of tyrosine. Remarkably, computational remodelling of AcmI and AcmL structures revealed significant similarity with the 3-D structures of type 1 O-methyltransferases from plants such as caffeic acid O-methyltransferase (COMT) and other phenylpropanoid methyltransferases. The relevance of 3-HK or 3-HA methylation in the actinomycin biosynthesis pathways of different actinomycetes is discussed.     

Actinomycin synthetases. Multifunctional enzymes responsible for the synthesis of the peptide chains of actinomycin

Two enzymes were purified from actinomycin-synthesizing Streptomyces chrysomallus which could be identified as peptide synthetases involved in the biosynthesis of actinomycin. Actinomycin synthetase II activates the first two amino acids of the peptide chains of the peptide lactone antibiotic, threonine and valine (or isoleucine), as thioesters via their corresponding adenylates. It is a single polypeptide chain of Mr 225,000. Similarly, actinomycin synthetase III activates proline, glycine, and valine (the remaining three amino acids in the antibiotic) as thioesters and is a single polypeptide chain of about Mr 280,000. It also carries the methyltransferase function(s) for N-methylation of thioesterified glycine and valine. In addition, it catalyzes the formation of cyclo(sarcosyl-N-methyl-L-valine) from glycine, L-valine, and S-adenosyl-L-methionine at the expense of ATP. Although the cell-free synthesis of the peptide lactone was not as yet accomplished, the data provide evidence that together with the 4-methyl-3-hydroxyanthranilic acid-activating enzyme (now designated as actinomycin synthetase I) all amino acid-activating protein components of the actinomycin-synthesizing enzyme complex are identified.     

Genetics of actinomycin C production in Streptomyces chrysomallus

Three distinct classes of mutations affecting the biosynthesis of actinomycin have been established in Streptomyces chyrsomallus by crossing various actinomycin-nonproducing mutants with each other by protoplast fusion. In crosses between members of different classes of mutations, actinomycin-producing recombinant progeny arose, whereas in crosses between members of the same class, no actinomycin-producing recombinants were seen. Biochemical examination of a number of mutants revealed that the expression of all actinomycin synthetases was reduced by about 1 order of magnitude in mutants belonging to class II. In mutants of class I, the specific activities of the actinomycin synthetases were comparable with those measured in their actinomycin-producing parents. Feeding experiments with 4-methyl-3-hydroxyanthranilic acid (4-MHA), the biosynthetic precursor of the chromophore moiety of actinomycin, with representative mutants of the three genetic classes revealed formation of actinomycin in minute amounts by mutants of class I. It is suggested that mutants belonging to class I are mutated at a genetic locus involved in the biosynthesis of 4-MHA. Mutants belonging to class II appear to carry mutations at a locus involved in the regulation of the expression of the actinomycin synthetases. The role of the locus in class III mutations could not be assigned. Mapping studies in S. chrysomallus based on conjugal matings revealed the chromosomal linkage of all three loci. Mutations belonging to classes I and III were closely linked. Their genetic loci could be localized in a map interval of the chromosomal linkage group which is significantly distant from the gene locus represented by mutations belonging to class II.     

Molecular characterization of the genes of actinomycin synthetase I and of a 4-methyl-3-hydroxyanthranilic acid carrier protein involved in the assembly of the acylpeptide chain of actinomycin in Streptomyces

Actinomycin synthetase I (ACMS I) activates 4-methyl-3-hydroxyanthranilic acid, the precursor of the chromophoric moiety of the actinomycin, as adenylate. The gene acmA of ACMS I was identified upstream of the genes acmB and acmC encoding the two peptide synthetases ACMS II and ACMS III, respectively, which assemble the pentapeptide lactone rings of the antibiotic. Sequence analysis and expression of acmA in Streptomyces lividans as enzymatically active hexa-His-fusion confirmed the acmA gene product to be ACMS I. An open reading frame of 234 base pairs (acmD), which encodes a 78-amino acid protein with similarity to various acyl carrier proteins, is located downstream of acmA. The acmD gene was expressed in Escherichia coli as hexa-His-fusion protein (Acm acyl carrier protein (AcmACP)). ACMS I in the presence of ATP acylated the purified AcmACP with radioactive p-toluic acid, used as substrate in place of 4-MHA. Only 10% of the AcmACP from E. coli was acylated, suggesting insufficient modification with 4'-phosphopantetheine cofactor. Incubation of this AcmACP with a holo-ACP synthase and coenzyme A quantitatively established the holo-form of AcmACP. Enzyme assays in the presence of ACMS II showed that toluyl-AcmACP directly acylated the thioester-bound threonine on ACMS II. Thus, AcmACP is a 4-MHA carrier protein in the peptide chain initiation of actinomycin synthesis.     

Actinomycin synthesis in Streptomyces antibioticus: enzymatic conversion of 3-hydroxyanthranilic acid to 4-methyl-3-hydroxyanthranilic acid.

A methyltransferase which utilizes 3-hydroxyanthranilic acid (HAA) as a substrate was identified in detergent-treated extracts of the bacterium Streptomyces antibioticus. The enzyme catalyzes the transfer of methyl groups from [14C]S-adenosylmethionine to HAA, but does not catalyze the methylation of 3-hydroxy-DL-kynurenine. Enzyme, substrate, time, and pH dependencies for the methyl transfer reaction were examined. Reaction products obtained from scaled-up reaction mixtures were fractionated by chromatography on Dowex 1, and the Dowex 1 fractions were examined by paper and thin-layer chromatography. One Dowex fraction was shown to contain a radioactive product with the chromatographic properties of 4-methyl-3-hydroxyanthranilic acid (MHA), a known intermediate in the biosynthesis of actinomycin. Available evidence indicates that the conversion of HAA to MHA is an early step in the biosynthesis of actinomycin by S. antibioticus and other actinomycin-producing streptomycetes.     

Biosynthesis of quinoxaline antibiotics: purification and characterization of the quinoxaline-2-carboxylic acid activating enzyme from Streptomyces triostinicus

A quinoxaline-2-carboxylic acid activating enzyme was purified to homogeneity from triostin-producing Streptomyces triostinicus. It could also be purified from quinomycin-producing Streptomyces echinatus. Triostins and quinomycins are peptide lactones that contain quinoxaline-2-carboxylic acid as chromophoric moiety. The enzyme catalyzes the ATP-pyrophosphate exchange reaction dependent on quinoxaline-2-carboxylic acid and the formation of the corresponding adenylate. Besides quinoxaline-2-carboxylic acid, the enzyme also catalyzes the formation of adenylates from quinoline-2-carboxylic acid and thieno[3,2-b]pyridine-5-carboxylic acid. No adenylates were seen from quinoline-3-carboxylic acid, quinoline-4-carboxylic acid, pyridine-2-carboxylic acid, and 2-pyrazinecarboxylic acid. Previous work [Gauvreau, D., & Waring, M. J. (1984) Can. J. Microbiol. 30, 439-450] revealed that quinoline-2-carboxylic acid and thieno[3,2-b]pyridine-5-carboxylic acid became efficiently incorporated into the corresponding quinoxaline antibiotic analogues in vivo. Together with the data described here, this suggests that the enzyme is part of the quinoxaline antibiotics synthesizing enzyme system. The enzyme displays a native molecular weight of 42,000, whereas in its denatured form it is a polypeptide of Mr 52,000-53,000. It resembles in its behavior actinomycin synthetase I, the chromophore activating enzyme involved in actinomycin biosynthesis [Keller, U., Kleinkauf, H., & Zocher, R. (1984) Biochemistry 23, 1479-1484].     

Molecular Cloning of the Actinomycin Synthetase Gene Cluster from Streptomyces chrysomallus and Functional Heterologous Expression of the Gene Encoding Actinomycin Synthetase II

The actinomycin synthetases ACMS I, II, and III catalyze the assembly of the acyl peptide lactone precursor of actinomycin by a nonribosomal mechanism. We have cloned the genes of ACMS I (acmA) and ACMS II (acmB) by hybridization screening of a cosmid library of Streptomyces chrysomallus DNA with synthetic oligonucleotides derived from peptide sequences of the two enzymes. Their genes were found to be closely linked and are arranged in opposite orientations. Hybridization mapping and partial sequence analyses indicate that the gene of an additional peptide synthetase, most likely the gene of ACMS III (acmC), is located immediately downstream of acmB in the same orientation. The protein sequence of ACMS II, deduced from acmB, shows that the enzyme contains two amino acid activation domains, which are characteristic of peptide synthetases, and an additional epimerization domain. Heterologous expression of acmB from the mel promoter of plasmid PIJ702 in Streptomyces lividans yielded a functional 280-kDa peptide synthetase which activates threonine and valine as enzyme-bound thioesters. It also catalyzes the dipeptide formation of threonyl–l-valine, which is epimerized to threonyl–d-valine. Both of these dipeptides are enzyme bound as thioesters. This catalytic activity is identical to the in vitro activity of ACMS II from S. chrysomallus.The actinomycins are a class of chromopeptide lactones produced by various Streptomyces strains. They contain two pentapeptide lactone rings attached to chromophoric 4,6-dimethylphenoxazinone-1,9-dicarboxylic acid (actinocin) in an amide-like fashion. Actinocin is formally derived from the compound 4-methyl-3-hydroxyanthranilic acid (4-MHA), but actually the bicyclic actinomycins arise from the oxidative condensation of two preformed monocyclic 4-MHA pentapeptide lactones (12). Previous investigations have revealed that the formation of the 4-MHA pentapeptide lactones is catalyzed by three actinomycin synthetases (ACMS I, II, and III) (13, 15). ACMS I (45 kDa) is a 4-MHA–AMP ligase which activates 4-MHA as adenylate. The five amino acids of the pentapeptide lactone ring of actinomycin (NH₂-cyclo[Thr–d-Val–Pro–N-methyl-Gly–N-methyl-Val] for actinomycin D) are assembled by ACMS II (280 kDa) and ACMS III (480 kDa) which from their properties belong to the class of peptide synthetases (13, 26, 27). ACMS II catalyzes the activation of threonine and valine. In the presence of ACMS I, which supplies 4-MHA–adenylate, 4-MHA–threonine and 4-MHA–threonyl–d-valine (via 4-MHA–threonyl–l-valine) are formed on the surface of ACMS II. In the absence of 4-MHA or ACMS I, purified ACMS II can synthesize both threonyl–l-valine and threonyl–d-valine, though to a lesser extent than the corresponding 4-MHA dipeptides can. The epimerization of valine is catalyzed by ACMS II at the acyl-dipeptide stage. An analysis of ACMS III suggests that it elongates the 4-MHA–Thr–d-Val dipeptide by successive incorporation of proline, N-methylglycine (sarcosine), and N-methyl-l-valine into the growing peptide chain (13). N-methylation is an additional feature of ACMS III. A total cell-free system for 4-MHA pentapeptide lactone synthesis is not available yet. Thus, it is not known how 4-MHA dipeptide transfer from ACMS II to ACMS III is accomplished, nor is the mechanism of lactone formation and release from the 4-MHA pentapeptide known.The available data indicate that ACMS II and ACMS III contain two- and three-amino-acid activation domains, respectively. It is known that activation domains of peptide synthetases are highly conserved in their sequences and are composed of a segment for amino acid adenylation and a segment for binding the activated amino acid as a thioester (17, 24, 25, 32). Thioester formation occurs via the thiol group of 4′-phosphopantetheine, which is a covalently bound cofactor of the activation domain. ACMS II and III both contain 4′-phosphopantetheine. In contrast, ACMS I has no 4′-phosphopantetheine cofactor, consistent with the finding that it does not form a thioester with 4-MHA. Data from previous work pointed instead to the formation of a 4-MHA thioester with ACMS II (26). In order to investigate the modular structure of the ACMSs and the reaction mechanisms in more detail, we set out to clone the ACMS genes from Streptomyces chrysomallus with oligonucleotide probes derived from partial sequences of ACMS I and II. We show that the genes of ACMS I and II and of a third peptide synthetase, most probably the gene of ACMS III (acmA, acmB, and acmC, respectively) are closely linked, forming a gene cluster. A total sequence determination of acmB and the characterization of the heterologously expressed functional active gene product confirm the significance of this peptide synthetase gene cluster.     

Control of the actinomycin biosynthetic pathway in and actinomycin resistance of Streptomyces spp.

Using actinomycin-producing and nonproducing strains of Streptomyces antibioticus, I studied several steps in the biosynthetic pathway of this antibiotic. Actinomycin-nonproducing strains derived after acriflavine or novobiocin treatment showed activity of kynurenine formamidase and phenoxazinone synthase as high as that of the parental strain, but these nonproducing strains failed to convert 4-methyl-3-hydroxy-anthranilic acid to actinomycin. In addition, accumulation of 4-methyl-3-hydroxyanthranilic acid (in the presence of D-valine) was not detected in the nonproducing isolates. Actinomycin-nonproducing strains derived after acriflavine treatment of Streptomyces parvulus showed a drastic decrease of resistance to the antibiotic. However these strains regained resistance after preincubation with a small amount of actinomycin D.     

The oxidation of 3-hydroxyanthranilic acid by Cu,Zn superoxide dismutase: mechanism and possible consequences

Cu,Zn SOD, but not Mn SOD, catalyzes the oxidation of 3-hydroxyanthranilic acid (3-HA) under aerobic conditions. In the absence of O2, the Cu(II) of the enzyme is reduced by 3-HA. One plausible mechanism involves the reduction of the active site Cu(II) to Cu(I), which is then reoxidized by the O2- generated by autoxidation of the anthranilyl or other radicals on the pathway to cinnabarinate. We may call this the superoxide reductase, or SOR, mechanism. Another possibility invokes direct reoxidation of the active site Cu(I) by the anthranilyl, or other organic radicals, or by the peroxyl radicals generated by addition of O2 to these organic radicals. Such oxidations catalyzed by Cu,Zn SOD could account for the deleterious effects of the mutant Cu,Zn SODs associated with familial amyotrophic lateral sclerosis and of the overproduction or overadministration of wild-type Cu,Zn SOD.     

In situ studies on AMP deaminase as a control system of the adenylate energy charge in yeasts

The role of AMP deaminase reaction in the stabilization of the adenylate energy charge was investigated using permeabilized yeast cells. The addition of P_i or Zn²⁺, which inhibits AMP deaminase, remarkably retarded the depletion of total adenylate pool and the recovery of the adenylate energy charge. Polyamine, an activator of the enzyme, decreased total adenylates, resulting in the enhanced recovery of the energy charge in situ. AMP deaminase can act as a regulatory enzyme in the system that stabilizes the adenylate energy charge in yeast cells under the conditions of severe metabolic stress.     

Chromophore activating enzyme involved in the biosynthesis of the mikamycin B antibiotic etamycin from Streptomyces griseoviridus

A 3-hydroxypicolinic acid activating enzyme from etamycin producing Streptomyces griseoviridus has been purified to apparent homogeneity. Etamycin is a member of mikamycin B antibiotics, chromopeptide lactones, which contain 3-hydroxypicolinic acid (3-HPA) as the chromophoric group. The enzyme catalyzes both the 3-HPA-dependent ATP-pyrophosphate exchange and the formation of 3-HPA adenylate from 3-HPA and ATP. SDS-polyacrylamide gel electrophoresis indicates that the enzyme is a single polypeptide chain with a Mr between 56,000 and 58,000. The molecular mass of the native enzyme was in the same range. In addition to 3-HPA, the enzyme catalyzes the formation of adenylates from picolinic acid, nicotinic acid, and 2-pyrazinecarboxylic acid. Nicotinic acid and picolinic acid when added externally to etamycin producing S. griseoviridus cultures gave rise to the formation of etamycin analogues each containing nicotinic acid or picolinic acid instead of the genuine 3-HPA. The data strongly suggest that the enzyme is involved in the biosynthesis of the chromopeptide lactone etamycin and possibly in that of other mikamycin B antibiotics.     

Actinomycin synthesis in Streptomyces antibioticus. Purification and properties of a 3-hydroxyanthranilate 4-methyltransferase

A methyltransferase, which utilizes 3-hydroxyanthranilic acid (HAA) as a substrate, has been purified to near homogeneity from 30-36-h mycelium of the bacterium Streptomyces antibioticus. The enzyme was obtained in approximately 20% yield with a purification of 130-fold. Polyacrylamide gel electrophoresis under denaturing conditions indicates that the enzyme is composed of a single subunit with Mr of about 36,000. On chromatography in 0.5 M NaCl, the enzyme displays a molecular weight of about 37,000. The specific activity of the enzyme in S. antibioticus mycelium is maximal between 30 and 36 h following inoculation of galactose/glutamic acid medium and, at those times post-inoculation, the specific activity is essentially the same in extracts of mycelium obtained from cultures grown on glucose rather than galactose as the carbon source. The enzyme activity is stimulated by Na2EDTA (in crude extracts) and by 2-mercaptoethanol and the methyltransferase shows a strong preference for HAA as substrate as compared with a number of HAA analogs. Thin layer chromatography of ethyl acetate extracts of large-scale incubation mixtures confirms that the product of the reaction is 4-methyl-3-hydroxyanthranilic acid. The reaction product was also a substrate for phenoxazinone synthase and was incorporated into actinomycin by S. antibioticus mycelium. Kinetic parameters for the methyltransferase reaction was determined.     

Substrate specificity and products of side-reactions catalyzed by jasmonate:amino acid synthetase (JAR1)

Jasmonate:amino acid synthetase (JAR1) is involved in the function of jasmonic acid (JA) as a plant hormone. It catalyzes the synthesis of several JA-amido conjugates, the most important of which appears to be JA-Ile. Structurally, JAR1 is a member of the firefly luciferase superfamily that comprises enzymes that adenylate various organic acids. This study analyzed the substrate specificity of recombinant JAR1 and determined whether it catalyzes the synthesis of mono- and dinucleoside polyphosphates, which are side-reaction products of many enzymes forming acyl approximately adenylates. Among different oxylipins tested as mixed stereoisomers for substrate activity with JAR1, the highest rate of conversion to Ile-conjugates was observed for (+/-)-JA and 9,10-dihydro-JA, while the rate of conjugation with 12-hydroxy-JA and OPC-4 (3-oxo-2-(2Z-pentenyl)cyclopentane-1-butyric acid) was only about 1-2% that for (+/-)-JA. Of the two stereoisomers of JA, (-)-JA and (+)-JA, rate of synthesis of the former was about 100-fold faster than for (+)-JA. Finally, we have demonstrated that (1) in the presence of ATP, Mg(2+), (-)-JA and tripolyphosphate the ligase produces adenosine 5'-tetraphosphate (p(4)A); (2) addition of isoleucine to that mixture halts the p(4)A synthesis; (3) the enzyme produces neither diadenosine triphosphate (Ap(3)A) nor diadenosine tetraphosphate (Ap(4)A) and (4) Ap(4)A cannot substitute ATP as a source of adenylate in the complete reaction that yields JA-Ile.     

Cloning, overexpression, and purification of novobiocic acid synthetase from Streptomyces spheroides NCIMB 11891

Novobiocic acid synthetase, a key enzyme in the biosynthesis of the antibiotic novobiocin, was cloned from the novobiocin producer Streptomyces spheroides NCIMB 11891. The enzyme is encoded by the gene novL, which codes for a protein of 527 amino acids with a calculated mass of 56,885 Da. The protein was overexpressed as a His(6) fusion protein in Escherichia coli and purified to apparent homogeneity by affinity chromatography and gel chromatography. The purified enzyme catalyzed the formation of an amide bond between 3-dimethylallyl-4-hydroxybenzoic acid (ring A of novobiocin) and 3-amino-4,7-dihydroxy-8-methyl coumarin (ring B of novobiocin) in an ATP-dependent reaction. NovL shows homology to the superfamily of adenylate-forming enzymes, and indeed the formation of an acyl adenylate from ring A and ATP was demonstrated by an ATP-PP(i) exchange assay. The purified enzyme exhibited both activation and transferase activity, i.e. it catalyzed both the activation of ring A as acyl adenylate and the subsequent transfer of the acyl group to the amino group of ring B. It is active as a monomer as determined by gel filtration chromatography. The reaction was specific for ATP as nucleotide triphosphate and dependent on the presence of Mg(2+) or Mn(2+). Apparent K(m) values for ring A and ring B were determined as 19 and 131 micrometer respectively. Of several analogues of ring A, only 3-geranyl-4-hydroxybenzoate and to a lesser extent 3-methyl-4-aminobenzoate were accepted as substrates.     

Development of a system for cloning a DNA fragment, containing the determinant of resistance to actinomycin for Streptomyces chrysomallus No.2--a producer of actinomycin C]

To study the modes of actinomycin biosynthesis and the mechanism responsible for resistance to the antibiotic producing S. chrysomallus No. 2, the authors undertook an examination and studies into the cloning system for gene(s) of resistance to actinomycin from a S. chrysomallus No. 2 actinomycin C producer and the cloning of a S. chrysomallus No. DNA fragment to the actinomycin-sensitive Streptomyces Sp. 26-115 H-I on the vector plasmid pIJ702. The cloning gave rise to actinomycin-resistant strains. The character of actinomycin resistance is inheritable in a steady fashion.     

The role of adenosine monophosphate nucleosidase in the regulation of adenine nucleotide levels in Azotobacter vinelandii during aerobic-anaerobic transitions

When cultures of Azotobacter vinelandii are made anaerobic the adenylate pool size remains constant or increases slightly while the adenylate energy charge decreases. Under these conditions, cell growth stops but the cells remain viable for at least 5 h with the decreased energy charge. The changes in the adenylate pool during the aerobic-anaerobic transition include: the formation of adenylates as a result of RNA degradation; the degradation of a portion of the excess AMP to form hypoxanthine by the sequential actions of AMP nucleosidase and adenine deaminase; an increase in the total adenylate pool which is stabilized at approximately 1.5 times the level in growing cells; and stabilization of the adenylate energy charge at a value near 0.3. The degradation of AMP is regulated by AMP nucleosidase, an allosteric enzyme which is activated by MgATP^2? and inhibited by P_i. The in vivo activity of AMP nucleosidase was estimated by measuring the rate of hypoxanthine formation in the culture or by measuring the activity of purified enzyme at the concentrations of AMP, ATP, and P_i found in the cells. The maximum estimated in vivo rate of AMP degradation was less than 3% of the catalytic capacity of AMP nucleosidase. Thus ample activity is present for rapid adjustments of the AMP levels in these cells. Expression of AMP nucleosidase catalytic activity is tightly controlled since high constant concentrations of intracellular AMP can be maintained for extended time periods at low adenylate energy charge values. Under these conditions controlled degradation of AMP can occur to maintain a constant AMP concentration.     

Phenoxazinone biosynthesis: accumulation of a precursor, 4-methyl-3-hydroxyanthranilic acid, by mutants of Streptomyces parvulus.

Mutants of Streptomyces parvulus that are blocked in the synthesis of the phenoxazinone-containing antibiotic, actinomycin, were isolated by the 'agar piece' method (after ultraviolet irradiation or treatment with 8-methoxypsoralen plus near-ultraviolet light). Radiolabelling experiments in conjunction with paper, thin-layer and column chromatography revealed that 4-methyl-3-hydroxyanthranilic acid (MHA) is a major metabolite accumulated by these mutants. Studies in vitro and in vivo provided evidence that MHA is a precursor of the phenoxazinone chromophore, actinocin. Normally MHA does not accumulate during growth or antibiotic synthesis by the parental strains. Protoplasts derived from the mutant strain AM5 synthesized MHA in significant amounts. A scheme is proposed for the biosynthesis of actinomycin D that accounts for the accumulation of MHA by the mutants.     

Rat pancreas adenylate cyclase V. Its presence in isolated rat pancreatic acinar cells.

(1) In order to determine the cellular localization of the secretin- and pancreozymin-sensitive adenylate cyclase in rat pancreas, the occurence of this enzyme system has been investigated in isolated pancreatic cells. (2) Digestion of rat pancreatic lobules with collagenase yields a preparation of isolated cells which upon differential morphological analysis appears to consist for 97% of acinar cells and to contain for fewer centro-acinar and ductal cells than undissociated lobules. (3) Expressed per mg protein, the isolated cells contain the same amount of DNA, chymotrypsin and lactic dehydrogenase as the undissociated tissue. The stimulated adenylate cyclase activity is nearly entirely recovered in the isolated acinar cells, as is also the case for the low Km adenosine 3',5-cyclic monophosphate phosphodiesterase activity and the adenosine 3',5'-cyclic monophosphate (cyclic AMP) content. Marked losses are noted for the basal adenylate cyclase and the high Km cyclic AMP phosphodiesterase activities. (4) Washing the isolated acinar cells in Krebs-Ringer bicarbonate medium containing 10 mM 1-methyl-3-isobutylxanthine causes a cyclic AMP level 2.6 times that in cells washed in Krebs-Ringer bicarbonate alone. The cyclic AMP level is further increased by subsequently incubating the cells for 10 min in the presence of 3-10(-7) M pancreozymin-C-octapeptide or secretin to values 1.7 or 4.7 times the control level in cells incubated for 10 min with 1-methyl-3-isobutylxanthine alone. (5) It is suggested that the adenylate cyclase of the acinar cells may be involved, with another factor, in the stimulation of enzyme secretion, whereas a ductular cyclase would function in the regulation of the bicarbonate-dependent fluid secretion.     

Functions of chloroplastic adenylate kinases in Arabidopsis

Adenosine monophosphate kinase (AMK; adenylate kinase) catalyses the reversible formation of ADP by the transfer of one phosphate group from ATP to AMP, thus equilibrating adenylates. The Arabidopsis (Arabidopsis thaliana) genome contains 10 genes with an adenylate/cytidylate kinase signature; seven of these are identified as putative adenylate kinases. Encoded proteins of at least two members of this Arabidopsis adenylate kinase gene family are targeted to plastids. However, when the individual genes are disrupted, the phenotypes of both mutants are strikingly different. Although absence of AMK2 causes only 30% reduction of total adenylate kinase activity in leaves, there is loss of chloroplast integrity leading to small, pale-looking plantlets from embryo to seedling development. In contrast, no phenotype for disruption of the second plastid adenylate kinase was found. From this analysis, we conclude that AMK2 is the major activity for equilibration of adenylates and de novo synthesis of ADP in the plastid stroma.