The overall goal of this research is to understand mechanisms used by organisms to defend themselves against photosensitizing compounds, compounds which are activated by light and, when photoactivated, are toxic to living cells. Specifically, this work will address the resistance of a group of fungal plant pathogens, Cercospora species, to the photosensitizing toxin cercosporin which they produce. Preliminary studies in this laboratory have demonstrated that the production of carotenoids and the presence of some cell surface factor(s) are responsible for cercosporin resistance. Of the two, the cell surface factor(s) appears to be of primary importance and will be the major emphasis of this work. Two major approaches will be used. In one, a heterologous transformation system will be used to identify genes responsible for cercosporin resistance. The fungus Neurospora crassa, which lacks the cell surface factor(s), will be used as a recipient for a Cercospora genomic library, and transformants will be screened for cercosporin resistance. In the second approach, interactions of the cercosporin molecule with sensitive (freshly isolated) and resistant (12 hour old) protoplasts will be studied. Attempts will be made to isolate a cercosporin-binding protein, and possible changes in the cercosporin molecule as it contacts the surface of a resistant cell will be studied by looking for alterations in the fluorescence emission spectra of cercosporin in contact with the surface of sensitive and resistant cells. This work has the potential to provide new and significant information relevant to a number of disciplines. Photosensitizers are important compounds in nature, and are used in medicine and agriculture, yet little is known about mechanisms by which certain organisms or cell types resist their effects. Characterization of the fungal cell surface factor(s) responsible for the resistance of this fungus to its own toxin therefore, will expand significantly our understanding of this phenomenon.