The imperfect filamentous fungus Scytalidium flavobrunneum produces a potent antifungal compound, 15-azasterol (2), and reddish brown pigment at the end of the exponential phase of growth and into the stationary phase (9). The ability to synthesize both the pigment and the azasterol were lost simultaneously, and revertants to production of the substances were never observed (R. J. Rodriguez, unpublished data). We have shown that there is a concomitant loss of fungal plasmid pSFB-1 with the antifungal agent and pigment (3). The 9.1-kilobase plasmid was isolated, purified, and characterized by restriction mapping.If S. flavo-brunneum is treated with 15-azasterol during active growth, growth inhibition results. Like most antifungal agents, 15-azasterol prevents normal sterol synthesis. In this case, the effect is to prevent reduction of the C14=C15 double bond, resulting in formation of the metabolically unacceptable sterol ignosterol (1). We assume, therefore, that production of the antifungal compound as a secondary metabolite is a physiological expedience by the organism to avoid self-intoxication.In this study, we achieved transformation of the nonazasterol-producing strain of S. flavo-brunneum to azasterol production with plasmid preparations from the azasterolproducing wild type, thus affirming the involvement of the plasmid in the formation of the antifungal agent. Here we present results of our study of the regulation of transcription and relative copy number of plasmid pSFB-1 during the culture cycle of S. flavo-brunneum. This is the first demonstration of a strong correlation of a specific biochemical trait to a native filamentous fungal plasmid. In addition, our results present insight into the control of expression of the plasmid in secondary metabolic regulation.
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