Candida albicans and some other pathogenic Candida species, when grown in a medium containing a protein as a sole source of nitrogen, secrete an acid proteinase. Culture supernatants were assayed for proteinase activity, and were also analysed by Western blotting with antibodies raised and affinity-purified against proteinase of C. albicans. Proteinases secreted by C. tropicalis and C. parapsilosis were antigenically related to that of C. albicans, but had different molecular masses. The proteinases secreted by C. lipolytica, C. rugosa and C. lusitaniae were not antigenically related. The kinetics of proteinase secretion by C. albicans were monitored by activity and by Western blotting. With BSA as the nitrogen source, proteinase secretion increased exponentially until about 16 h. Culture supernatants of BSA-grown cultures accumulated proteinase to about a 1000-fold higher level than those of ammonium-sulphate-grown cultures. In vitro labelling experiments showed that proteinase was not detectably accumulated in the cells, but was secreted immediately after synthesis. Immunoprecipitation of in uitro translated poly(A)-containing RNA identified a putative pre-protein of about 54 kDa. As well as BSA, other proteins (haemoglobin, ovalbumin, histone), peptone and tryptone, when used as nitrogen sources, could induce proteinase, but to different levels. When Casamino acids or an amino acid mixture (equivalent to the composition of BSA) was used as nitrogen source, no induction was observed. Ammonium sulphate, or any other ammonium salt, repressed secretion of proteinase.
Candida albicans secretes an acid proteinase when grown with a protein as a sole nitrogen source. The gene encoding this proteinase was isolated from a genomic expression library of C. albicans constructed in Agtll by screening with antiproteinase antibodies. The affinity-purified antibodies used to verify the clones are monospecific; these do not cross-react with any other protein in the culture supernatants or crude extracts of C. albicans but strongly react with fusion proteins encoded by recombinant clones, revealing that these are true proteinase clones. Genomic Southern blot analysis shows that the proteinase gene is present at a unique locus and that there is no other closely related gene in the C. albicans genome. The proteinase gene probe identified two transcripts on Northern blots (RNA blots), which are present at a much higher level in C. albicans cells induced for proteinase secretion than in uninduced cells. The aspartyl proteinase gene reported earlier (T. J.
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