Using a Tn7 transposon library of Candida albicans, we have identified a mutant that exhibited sensitivity in drop plate assays to oxidants such as menadione and hydrogen peroxide. To verify the role of the mutated gene in stress adaptation, null mutants were constructed and phenotypically characterized. Because of its apparent functions in growth and oxidant adaptation, we have named the gene GOA1. Goa1p appears to be unique to the CTG subclade of the Saccharomycotina, including C. albicans. Mutants of C. albicans lacking goa1 (strain GOA31) were more sensitive to 6 mM H 2 O 2 and 0.125 mM menadione than the wild type (wt) or a genereconstituted (GOA32) strain. The sensitivity to oxidants correlated with reduced survival of the GOA31 mutant in human neutrophils and avirulence compared to control strains. Other phenotypes of GOA31 include reduced growth and filamentation in 10% serum, Spider, and SLAD agar media and an inability to form chlamydospores. Since Goa1p has an N-terminal mitochondrion localization site, we also show that green fluorescent protein-tagged Goa1p shows a mitochondrionlike distribution during oxidant or osmotic stress. Further, the inability of GOA31 to grow in medium containing lactate, ethanol, or glycerol as the sole carbon source indicates that the mitochondria are defective in the mutant. To determine how Goa1p contributes to mitochondrial function, we compared the wt, GOA32, and GOA31 strains for mitochondrial electrical membrane potential, respiration, and oxidative phosphorylation. We now show that GOA31, but not the wt or GOA32, had decreased respiration and mitochondrial membrane potential such that mutant cells are unable to drive oxidative phosphorylation. This is the first report in C. albicans of a respiratory defect caused by a loss of mitochondrial membrane potential.

Regulation of hyphal morphogenesis in Candida albicans can occur through quorum sensing (QS). A QS signal, farnesol, is produced during high-density growth and inhibits morphogenesis. However, the signal transduction pathway that regulates QS is unknown. Here, we show that a C. albicans mutant lacking Chk1p but not either the Sln1p or the Nik1p histidine kinase is refractory to the inhibitory effect of farnesol both in cell suspension and during the formation of a biofilm. This study is the first to demonstrate a role for a two-component signal transduction protein in QS by a eukaryotic organism.Cell density is a critical factor in the regulation of Candida albicans hyphal morphogenesis. At a density of Ͼ10 6 cells/ml, yeast cells do not shift (germinate) to hyphae or do so at low frequencies, while at a density of Ͻ10 6 cells/ml, germination occurs (3). The relationship between cell density and new gene transcription (hyphal morphogenesis) resembles quorum sensing (QS) in some bacteria (14). Recent observations indicate that a QS system operates in C. albicans and that the isoprenoid farnesol is the QS autoinducer signal (12). Cells exposed to farnesol do not germinate, even at low cell densities. However, the regulatory and signal transduction events that direct QS are unknown, not only for C. albicans but for other fungi and eukaryotes in general. In some bacteria, two-component signaling regulates QS. Since C. albicans has several two-component signal proteins that are critical to a number of processes, including cell wall biosynthesis, adaptation to stress conditions, and virulence, our rationale was that farnesol sensing could be mediated through two-component proteins. C. albicans has three hybrid-histidine kinases, two of which have orthologues in Saccharomyces cerevisiae (Sln1p) and Neurospora crassa (Nik1p) that are presumed to play a role in an osmotic stress response (1,15,19,20). The third histidine kinase, Chk1p, has some similarity to two Schizosaccharomyces pombe proteins, Mak2p and Mak3p, which are known to function as sensors for oxidative stress (2, 5). In addition to the histidine kinases, C. albicans has two response regulator proteins, Ssk1p and Skn7p, whose S. cerevisiae homologs act downstream of the Sln1p histidine kinase (11). In C. albicans, Ssk1p and Skn7p function in the adaptation of cells to oxidant stress, while Ssk1p, in addition, regulates the expression of structural cell wall proteins and negatively regulates Chk1p expression (7, 18).The C. albicans strains used for this study have been described previously (4, 6, 9, 20). Unless noted, cells were routinely cultured in YPD (1% yeast extract, 2% dextrose, 2% peptone) or YNB (0.67% yeast nitrogen base [pH 7.0], 50 mM glucose) at 30°C. To assess whether the two-component signal transduction proteins of C. albicans play a role in QS, all strains (see Table 1) were first cultured overnight at 30°C in YPD. Subsequently, the cells were washed twice and then inoculated into 10 ml of prewarmed medium 199 (pH 7.5) with or without 250 M t...

No abstract

No abstract