Biosynthesis of sporogenic factors (psi) in the filamentous fungus Aspergillus nidulans

The filamentous fungus Aspergillus nidulans produces an ethyl acetate extractable activity that prematurely induces the sexual phase when applied to a confluent lawn of this fungus. This biological activity was named psi for precocious sexual inducer and was found to be due, at least in part, to three hydroxylated analogs of linoleic acid. PsiB is 8-hydroxy linoleic acid: psiC is 5,8-dihydroxy linoleic acid: and psiA is the lactonized ester of psiC. The structural similarity between linoleic acid and psi molecules suggested that linoleic acid is the precursor of psi compounds. Psi compounds were labeled from ;An in vitro system was developed that converts linoleic acid into two of the three known psi compounds, psiB and psiC. This system was used to characterize the hydroxylating enzyme(s) that form psi compounds. Results suggest that two enzymatic activities are responsible for the formation of psi compounds. Moreover, metabolic feeding studies showed that psiB is not a precursor of psiC. Collectively, these results suggest that psi compounds are formed via two independent pathways. The conversion of linoleic acid into psi compounds requires an aerobic environment. Fatty acid oxygenase inhibitors were tested for their ability to affect the reaction. Lipoxygenase inhibitors prevented the conversion of linoleic acid into psi species whereas cyclooxygenase and cytochrome P450 inhibitors were relatively ineffective. This suggests that a "lipoxygenase like" mechanism may be responsible for the formation of psi compounds. The in vitro system was also used to determine if other Aspergillus species convert linoleic acid into psi compounds. Results show that linoleic acid to psi converting activity correlates with ascospore production in Aspergillus species.;Interestingly, psiB is identical to laetisaric acid which is produced by Laetisaria arvalis. This compound was demonstrated to protect plants from Pythium ultimum, a common plant pathogen that causes black root disease. The described A. nidulans in vitro system may also provide an excellent system for the production of laetisaric acid (psiB).