A survey of cytosine and cytidine salvage in fungi

Twenty-five fungi, including representatives from each of the four major classes, were surveyed with regard to their ability to utilize exogenous cytosine and cytidine. Two assay methods were employed: incorporation of radiolabeled precursors and sensitivity to 5-fluorinated structural analogs. The following taxonomic correlations were found: (i) the Ascomycetes utilize exogenous cytidine and/or cytosine; (ii) the lower fungi utilize cytidine but not cytosine; (iii) the Basidiomycetes utilize neither; and (iv) the Dueteromycetes, with some exceptions, closely resemble the Ascomycetes. Additionally, natural resistance to 5-fluorocytosine via both defective transport and pyrimidine excretion was observed.