[Quantitative determination of the enzyme cyclopenase in conidiospores of Penicillium cyclopium Westling and P. viridictatum Westling]

Enzymatic one-step ring contraction for quinolone biosynthesis

The 6,6-quinolone scaffolds on which viridicatin-type fungal alkaloids are built are frequently found in metabolites that display useful biological activities. Here we report in vitro and computational analyses leading to the discovery of a hemocyanin-like protein AsqI from the Aspergillus nidulans aspoquinolone biosynthetic pathway that forms viridicatins via a conversion of the cyclopenin-type 6,7-bicyclic system into the viridicatin-type 6,6-bicyclic core through elimination of carbon dioxide and methylamine through methyl isocyanate.

Viridicatin is a fungal alkaloid. Here, the authors identify and characterize the cyclopenase that catalyzes the last step of its biosynthesis in Aspergillus nidulans, the conversion of cyclopenin to viridicatin, and find that the reaction proceeds via an unusual elimination mechanism.

Elucidation of Biosynthetic Pathways of Natural Products

During the last decade, we have revealed biosynthetic pathways responsible for the formation of important and chemically complex natural products isolated from various organisms through genetic manipulation. Detailed in vivo and in vitro characterizations enabled elucidation of unexpected mechanisms of secondary metabolite biosynthesis. This personal account focuses on our recent efforts in identifying the genes responsible for the biosynthesis of spirotryprostatin, aspoquinolone, Sch 210972, pyranonigrin, fumagillin and pseurotin. We exploit heterologous reconstitution of biosynthetic pathways of interest in our study. In particular, extensive involvement of oxidation reactions is discussed. Heterologous hosts employed here are Saccharomyces cerevisiae, Aspergillus nidulans and A. niger that can also be used to prepare biosynthetic intermediates and product analogs by engineering the biosynthetic pathways using the knowledge obtained by detailed characterizations of the enzymes. (998 char.).

Uric acid is a genuine metabolite of Penicillium cyclopium and stimulates the expression of alkaloid biosynthesis in this fungus

On searching for endogenous, low-molecular-weight effectors of benzodiazepine alkaloid biosynthesis in Penicillium cyclopium uric acid was isolated from ethanolic or autoclaved mycelial extracts of this fungus. The isolation was based on a three-step high-pressure liquid chromatography procedure guided by a microplate bioassay, and uric acid was identified by mass spectrometry and the uricase reaction. Conidiospore suspensions that were treated with this compound during the early phase of outgrowth developed emerged cultures with an enhanced rate of alkaloid production. Uric acid treatment did not increase the in vitro measurable activity of the rate-limiting biosynthetic enzyme, cyclopeptine synthetase. However, these cultures displayed a reduced rate of uptake of the alkaloid precursor L-phenylalanine into the vacuoles of the hyphal cells as assayed in situ. It is suggested that the depressed capacity of vacuolar uptake caused by the contact of outgrowing spores with uric acid liberated from hyphal cells results in an enhanced availability of the precursor L-phenylalanine in the cytoplasm and thus accounts at least in part for the increase in alkaloid production.

Cyclopeptine synthetase activity in surface cultures of Penicillium cyclopium

Cyclopeptine synthetase, the key enzyme of benzodiazepine alkaloid biosynthesis in Penicillium cyclopium forms cyclo-(anthranoyl-phenylalanyl) from anthranilic acid, L-phenylalanine, the methyl group of L-methionine and ATP. The following in vitro measurable partial activities of the enzyme system were followed during the development of P. cyclopium: anthranilic acid and L-phenylalanine adenylyltransferase activities, and the ability for thioester-binding of L-phenylalanine to the enzyme protein. These activities became measurable at the beginning of the idiophase and reached a maximum 6 days after inoculation, i.e., the pattern of activity was similar to that of the other enzymes participating in the biosynthesis of the benzodiazepine alkaloids indicating that the activities of all enzymes of the pathway were coordinatedly expressed. Inhibitor experiments indicated that 48-55 h after inoculation a preprotein of anthranilic acid adenylyltransferase was formed, which later on became activated by a hitherto unknown mechanism.

Nuclear inheritance of the biosynthesis of cyclopenin and cyclopenol in Penicillium cyclopium

Balanced heterokarions were grown from Penicillium cyclopium aux-glu 1, a glutamic acid auxotroph producing benzodiazepine alkaloids of the cyclopenin-cyclopenol group, and P. viridicatum aux-met 1, a methionine auxotroph forming these alkaloids in traces only. In contrast to the hyphae of the parent strains, the hyphae of the heterokarions were dark orange-brown and grew well on media without the auxotrophic factors. In surface cultures they synthesized the benzodiazepine alkaloids cyclopenin and cyclopenol in amounts similar to those formed by the hyphae of P. cyclopium aux-glu 1. From the monokariotic conidiospores of the heterokarions homokariotic daughter strains were obtained which were similar to the parent strains in every respect. Hence no exchange of features of cyclopenin-cyclopenol biosynthesis took place between the parent strains at the stage of the heterokarion. This result indicates that the formation of cyclopenin and cyclopenol in P. cyclopium aux-glu 1 and the nearly complete lack of biosynthesis of these compounds in P. viridicatum aux-met 1 is encoded within the nucleus and is not influenced by plasmic genetic material.

[Selection and characterization of Penicillium cyclopium mutants with altered developmental program]

After treatment with NNMG, NaNO2 or UV-light of conidiospores of Penicillium cyclopium strain SM 72 variant strains (mutants) with altered developmental programme were selected. Additionally from a methionine-auxotrophic mutant of P. cyclopium prototrophic revertants were prepared. Investigation of the alkaloid metabolism and other idiophase processes has shown that these mutant strains can be divided into two groups (cf. table 1): a) Mutants with a depression of all idiophase features. The defects of these strains presumably affect central regulatory processes which, as in strain rev-met 83a, can be reversed spontaneously by an one-step mechanism and b) mutants defective in certain parts of the idiophase programme only, demonstrating that there is a certain autonomy in regulation of the individual parts of the programme.