Lichens produce a diverse array of bioactive secondary metabolites, many of which are unique to the organisms. Their potential applications, however, are limited by their finite sources and the slow-growing nature of the organisms in both laboratory and environmental conditions. This thesis set out to investigate polyketide synthase genes in lichens, with the ultimate goal of providing a sustainable source of lichen natural products to support these applications. To expand the diversity of PKS genes that could be detected in lichens, new degenerate primers targeting ketoacylsynthase (KS) domains of specific clades of PKS genes have been developed and tested on various lichen samples. Using these primers, 19 KS domains from various lichens were obtained. Phylogenetic analysis of the KS domains was used to infer the function of the PKS genes based on the predicted PKS domain architecture and chemical analysis by TLC and/or HPLC. KS domains from PKS clades not previously known in lichens were identified; this included the clade III NR (non-reducing)-PKSs, PR (partially reducing)-PKSs and HR (highly reducing)-PKSs. The discovery of clade III NR-PKSs with C-methyltransferase (CMeT) domain and their wide occurrence in lichens was especially significant. Based on the KS domain phylogenetic analysis and compounds detected in the individual lichens, the clade III NR-PKSs were hypothesized to be involved in the biosynthesis of β-orsellinic acid and methylphloroacetopheno ne - the monoaromatic precursors for many lichen coupled phenolic compounds, such as β-orcinol depsides/depsidones and usnic acid. A strategy has been developed to isolate clade III NR-PKSs directly from environmental lichen DNA using clade III NR-type KS amplified from the degenerate primers (NR3KS-F/R) as homologous probes. Another pair of degenerate primers specific to the CMeT domain of NR-PKSs has also been developed to facilitate the cloning and probing of new clade III NR-PKS genes in lichens. A clade III NR-PKS gene (xsepks1) from X. semiviridis was cloned successfully. This is the first report of the isolation of a full-length PKS gene from environmental lichen DNA. The domain architecture of xsepks1 is KS-AT-ACP-CMeT, as expected for a clade III NR-PKS, suggesting that the newly developed clade-specific primers are useful for cloning new clade III NR-PKS genes and that KS domain phylogenetic analysis can predict the functional domains in PKSs. Attempts were made to characterize the function of xsepks1 by heterologous expression in Aspergillus species. Both A. nidulans (transformed with 5´partial xsepks1 including native promoter) and A. oryzae (transformed with full-length xsepks1 under the regulation of starch-inducible amyB promoter) were tested as potential hosts for the expression of lichen PKS genes. Transcriptional analysis showed that A. nidulans could potentially utilize the lichen PKS gene promoter and both fungal hosts could splice the introns of a lichen PKS gene. Several compounds unique to the A. oryzae transformants carrying xsepks1 were detected, but they could not be reproduced in subsequent fermentations even though the gene was transcribed into mRNA. None of the expected products (β-orsellinic acid, methylphloroacetophenone or similar methylated monoaromatic compounds) was detected in A. oryzae transformants, and the function of xsepks1 remains to be determined. The other clade III NR-PKS genes detected in X. semiviridis cou ld also be responsible for the biosynthesis of β-orsellinic acid or methylphloroacetophenone, as precursors of the major secondary metabolites detected in X. semiviridis (i.e. fumarprotocetraric acid, succinprotocetraric acid and usnic acid). Overall, the work in this thesis demonstrated the prospect of using a molecular approach to access the lichen biosynthetic potential without going through the cumbersome culturing stage.

Aspergillus westerdijkiaem qui est récemment démembré d'A. ochraceus est un producteur principal de plusieurs composés de type polycétone d'importance économique. Ces composés incluent l’ochratoxin A, mellein, l'acide penicillique, asperlactone et l’isoasperlactone et quelques intermédiaires comme l'acide 6- methylsalicylique et l’acide orsellinique. La biosynthèse de ces métabolites est catalysée par un groupe d'enzymes connues comme la polycétone synthases (PKSs). Ce travail a été visé pour cloner et a caractérisé fonctionnellement les différentes genes des PKS i.e. aoks1, aolc35-12 et aomsas, et de genes de polyketide synthases-non ribosomal peptide synthase (PKS-NRPS) i.e. aolc35-6, chez A. westerdijkiae. Ces gènes ont été inactivés par l'insertion du gène d’hygromycine B phosphotransferase d’Escherichia coli dans le génome d'A. westerdijkiae, pour obtenir les mutantes ao?ks1, ao?lc35-12, ao?msas et ao?lc35-6. Les mutants ao?ks1 et ao?lc35-12 ont été trouvés déficients dans la biosynthèse d’ochratoxin A, mais produisaient encore mellein. À notre connaissance, c’est la première fois que nous avons caractérisé les gènes impliquées dans la biosynthèse d’OTA, sachant que mellein, qui était proposé dans la littérature comme un intermédiaire, joue a cune role dans la biosynthesis de l'OTA. Ensuite le mutant ao?msas n'a pas seulement perdu la capacité de produire isoasperlactone et asperlactone, mais aussi il ne produit pas l’intermédiaire acide 6-methylsalicylique. Basé sur les expériences de la caractérisation génétique et de complémentation chimiques, nous avons proposé un shéma hypothétique de la biosynthèse d’asperlactone et isoasperlactone dans lequel l'acide 6-methylsalicylique, diepoxide et aspyrone jouent le rôle d’intermédiaires. La techniques de gène knock-out et de la reverse transcription PCR (RT-PCR) ont montré que seulle gène de type PKS-NRPS « aolc35-6 » identifié chez A. westerdijkiae codant pour un intermédiaire inconnu(s) qui pourrait inciter l'expression de gène aomsas et un gène impliqué dans la biosynthèse d'acide orsellinique et d'acide penicillique. / Aspergillus westerdijkiaem which is recently dismembered from A. ochraceusm is the principal producer of several economically important polyketide metabolites. These metabolites include ochratoxin A, mellein, penicillic acid, asperlactone and isoasperlactone and some intermediates like orsellinic acid and 6-methylsalicylic acid. The biosynthesis of these metabolites is catalyzed by a group of enzymes known as polyketide synthases (PKSs). This work was aimed to clone and functionally characterized various PKS i.e. aoks1, aolc35-12 and aomsas, and polyketide synthasesnon ribosomal peptide synthase (PKS-NRPS) genes i.e. aolc35-6, in A. westerdijkiae. These genes were inactivated by the insertion of Escherichia coli hygromycin B phosphotransferase gene in the genome of A. westerdijkiae to obtain ao?ks1, ao?lc35-12, ao?msas and ao?lc35-6 mutants. ao?ks1, ao?lc35-12 mutants were found deficient in ochratoxin A biosynthesis but are still producing mellein. To our knowledge, we for the first time characterized a gene involved in OTA biosynthesis, with the information about mellein which was proposed in the literature to be an intermediate OTA. Further ao?msas mutant not only lost the capacity to produce isoasperlactone and asperlactone but also the intermediate nature product 6-methylsalicylic acid. Based on the genetic characterization and chemical complementation experiments, we have proposed a hypothetical pathway mentioning that 6-methylsalicylic acid, diepoxid and aspyrone are intermediates of isoasperlactone and asperlactone. Gene knockout technique and reverse transcription PCR (RT-PCR) shown that the only PKS-NRPS gene aolc35-6 so far identified in A. westerdijkiae encoding certain unknown intermediate(s) which induces the expression of aomsas gene and a gene involved in the biosynthesis of orsellinic acid and penicillic acid.