Genetics and Biochemistry of Secondary Metabolism

The affinity purification and characterization of a dehydrogenase from Aspergillus parasiticus involved in aflatoxin B1 biosynthesis

A two step scheme has been developed for the purification of a dehydrogenase from mycelia of 84 hours old Aspergillus parasiticus (1-11-105 Wh 1), which catalyzes the conversion of norsolorinic acid (NA) to averantin (AVN). The dehydrogenase was purified from cell-free extracts using reactive green 19-agarose and norsolorinic acid-agarose affinity chromatography. The latter affinity matrix was synthesised by attaching norsolorinic acid to omega-aminohexylagarose. The purified protein was shown to be homogenous on non-denaturing polyacrylamide gel electrophoresis. A final purification of 215-fold was achieved. Results of gel filtration chromatography indicated the approximate molecular mass of the native protein to be 140,000 daltons. The isoelectric point of the protein was about 5.5 as determined by chromatofocusing. The reaction catalyzed by the dehydrogenase was optimum at pH 8.5 and between 25 degrees to 35 degrees C. The Km of the enzyme for NA and NADPH was determined to be 3.45 microM and 103 microM respectively.

Identification and characterization of genes for a second anthranilate synthase in Pseudomonas aeruginosa: interchangeability of the two anthranilate synthases and evolutionary implications

Two anthranilate synthase gene pairs have been identified in Pseudomonas aeruginosa. They were cloned, sequenced, inactivated in vitro by insertion of an antibiotic resistance gene, and returned to P. aeruginosa, replacing the wild-type gene. One anthranilate synthase enzyme participates in tryptophan synthesis; its genes are designated trpE and trpG. The other anthranilate synthase enzyme, encoded by phnA and phnB, participates in the synthesis of pyocyanin, the characteristic phenazine pigment of the organism. trpE and trpG are independently transcribed; homologous genes have been cloned from Pseudomonas putida. The phenazine pathway genes phnA and phnB are cotranscribed. The cloned phnA phnB gene pair complements trpE and trpE(G) mutants of Escherichia coli. Homologous genes were not found in P. putida PPG1, a non-phenazine producer. Surprisingly, PhnA and PhnB are more closely related to E. coli TrpE and TrpG than to Pseudomonas TrpE and TrpG, whereas Pseudomonas TrpE and TrpG are more closely related to E. coli PabB and PabA than to E. coli TrpE and TrpG. We replaced the wild-type trpE on the P. aeruginosa chromosome with a mutant form having a considerable portion of its coding sequence deleted and replaced by a tetracycline resistance gene cassette. This resulted in tryptophan auxotrophy; however, spontaneous tryptophan-independent revertants appeared at a frequency of 10(-5) to 10(6). The anthranilate synthase of these revertants is not feedback inhibited by tryptophan, suggesting that it arises from PhnAB. phnA mutants retain a low level of pyocyanin production. Introduction of an inactivated trpE gene into a phnA mutant abolished residual pyocyanin production, suggesting that the trpE trpG gene products are capable of providing some anthranilate for pyocyanin synthesis.

Formation and physiological role of biosurfactants produced by hydrocarbon-utilizing microorganisms

Microbial growth on water-insoluble carbon sources such as hydrocarbons is accompanied by metabolic and structural alterations of the cell. The appearance of surface-active compounds (biosurfactants) in the culture medium or attached to the cell boundaries is often regarded as a prerequisite for initial interactions of hydrocarbons with the microbial cell. Under this point of view, biosurfactants produced by hydrocarbon-utilizing microorganisms, their structures and physico-chemical properties are reviewed. The production of such compounds is mostly connected with growth limitation in the late logarithmic and the stationary growth phase, in which specific enzymes are induced or derepressed. Addition of purified biosurfactants to microbial cultures resulted in inhibitory as well as in stimulatory effects on growth. Therefore, a more differentiated view of microbial production of surface-active compounds is proposed. Biosurfactants should not only be regarded as prerequisites of hydrocarbon uptake, but also as secondary metabolic products.

Enzymes in aflatoxin B1 biosynthesis: strategies for identifying pertinent genes

Recent work on the aflatoxin biosynthetic pathway is reviewed, with special emphasis on the enzymes of the late stages of the pathway involving conversion of sterigmatocystin (ST) to aflatoxin B1 (AFB1) through an O-methylsterigmatocystin intermediate. Two enzyme activities were discovered in subcellular fractions of cell-free extracts of a mutant strain of Aspergillus parasiticus (SRRC 163): 1) A post-microsomal methyltransferase (MT) catalyzed conversion of ST to OMST, and 2) a microsomal-associated activity (oxido-reductase) converted OMST to AFB1. The 168 KDa, anionic MT was purified to homogeneity and characterized (two subunits, 110 KDa and 58 KDa). Preliminary evidence indicated the presence of a cationic isozyme of the MT in mycelial extracts. The oxido-reductase has been partially purified and characterized. Polyclonal antibodies were prepared to the anionic MT and the enzyme's amino acid composition determined. A cDNA library has been constructed from mRNA isolated from Aspergillus parasiticus mycelia during the onset of AFB1 biosynthesis for the purpose of identifying the genes responsible for aflatoxin biosynthesis.

Leucine auxotrophy specifically alters the pattern of trichothecene production in a T-2 toxin-producing strain of Fusarium sporotrichioides

The biosynthetic pathway for trichothecenes in the filamentous fungus Fusarium sporotrichioides NRRL 3299 has been further characterized. Experiments using the techniques of mutational analysis and the incorporation of radiolabeled precursors indicated that leucine is a direct precursor to the isovalerate moiety present in the trichothecene, T-2 toxin. Analysis of trichothecene production in a UV-induced leucine auxotroph also revealed the existence of a branched biosynthetic pathway which results in the coproduction of T-2 toxin and the T-2 toxin analogs neosolaniol, 8-isobutyryl-neosolaniol, and 8-propionyl-neosolaniol. Leucine limitation imposed by the leucine auxotroph simultaneously led to underproduction of T-2 toxin and overproduction of these T-2 toxin analogs, which are produced in small amounts by the wild-type parent. Furthermore, it was shown that the ratio of T-2 toxin to T-2 toxin analogs produced by the leucine auxotroph can be modulated by the concentration of leucine in the medium. These results suggest that the four trichothecenes mentioned above are derived from a common intermediate and that there is competition for this intermediate among the branched pathways leading to these four cometabolites.

Identification of O-methylsterigmatocystin as an aflatoxin B1 and G1 precursor in Aspergillus parasiticus

An isolate of Aspergillus parasiticus CP461 (SRRC 2043) produced no detectable aflatoxins, but accumulated O-methylsterigmatocystin (OMST). When sterigmatocystin (ST) was fed to this isolate in a low-sugar medium, there was an increase in the accumulation of OMST, without aflatoxin synthesis. When radiolabeled [14C]OMST was fed to resting mycelia of a non-aflatoxin-, non-ST-, and non-OMST-producing mutant of A. parasiticus AVN-1 (SRRC 163), 14C-labeled aflatoxins B1 and G1 were produced; 10 nmol of OMST produced 7.8 nmol of B1 and 1.0 nmol of G1, while 10 nmol of ST produced 6.4 nmol of B1 and 0.6 nmol of G1. A time course study of aflatoxin synthesis in ST feeding experiments with AVN-1 revealed that OMST is synthesized by the mold during the onset of aflatoxin synthesis. The total amount of aflatoxins recovered from OMST feeding experiments was higher than from experiments in which ST was fed to the resting mycelia. These results suggest that OMST is a true metabolite in the aflatoxin biosynthetic pathway between sterigmatocystin and aflatoxins B1 and G1 and is not a shunt metabolite, as thought previously.

Theoretically possible yields of microbial secondary products

Balance equations for some representatives of the main biogenetic groups of secondary metabolites were formulated which provide the known and probable reactions within the biosynthetic routes. The calculated yields Yp/s max with respect to the usual substrate glucose range between 0.37 and 0.9 and, obviously, they agree well with the theoretical yields of biochemically related parts of cell dry mass. The sugar derivatives correspond to carbohydrates, the beta-lactams and shikimate derivatives to proteins, and finally, the polyketides and mevalonate derivatives to lipids. From this stoichiometric point of view the secondary metabolism thus seems to possess the same biosynthetic efficiency as adequate parts of primary metabolism under the conditions of retarded growth. The introduction of the Yp/s max values into an equation for substrate consumption regarding the growth, maintenance and product formation (van der Beek and Roels 1984) delivered yields which demonstrated that more than 70% of the glucose flowed into byproducts or waste metabolites, respectively.