Abstract
The two-component histidine kinase Chk1p of Candida albicans has been implicated in the regulation of cell wall biosynthesis. Deletion of CHK1 results in avirulence that in part may be due to the increased sensitivity of mutant strains to polymorphonuclear leukocytes. The mutant also does not adhere to human oesophageal tissue in vitro, probably as a consequence of its altered cell wall. In the current study, a CHK1 promoter-lacZ reporter (CHK1prlacZ) construct was expressed in wild-type C. albicans strain CAI4 and in two-component signal transduction mutants to determine the effect of environmental stress conditions on the regulation of CHK1 and the co-regulatory activities among these proteins. It is shown that lacZ expression varied according to the type of growth conditions and incubation time; expression was also influenced by the strain background. lacZ expression in CAI4 was greater at 37 degrees C and at a pH of 3.5 and in the presence of 4 mM H2O2, 0.1 mM menadione, 10 % serum or 1.5 M NaCl compared to cells grown at 30 or 42 degrees C. The increases in expression were time-dependent and not observed until cells were incubated for 120 min in these conditions (P < 0.05). As a correlate of the increase in transcription of CHK1-lacZ in the presence of H2O2, the chk1 mutant was more sensitive than wild-type and revertant cells to H2O2 in vitro. In addition to strain CAI4, we also measured CHK1p-lacZ reporter activity of mutants deleted in genes encoding other two-component proteins such as the response regulator gene SSK1, the histidine kinases, SLN1 and NIK1, and the HOG1 MAP kinase. Of these proteins, Ssk1p and Sln1p are presumed to mediate phosphotransfer to the HOG1 [hyperosmotic glycerol] MAP kinase pathway during oxidative and perhaps osmotic stress in C. albicans. Compared to strain CAI4, lacZ reporter activity increased significantly in the ssk1 mutant under all growth conditions after a 10 and 120 min incubation (P < 0.0001). lacZ expression in the ssk1 mutant was less at 42 degrees C compared to all other growth conditions (P < 0.05). Furthermore, lacZ reporter activity also increased in the hog1 mutant of C. albicans. These data suggest that SSK1 and HOG1 indirectly or directly negatively regulate CHK1 under most growth conditions tested. In the sln1 mutant, downregulation of CHK1 was observed in all growth conditions compared to strain CAI4 (P < 0.05), while regulation of lacZ in the nik1 mutant was similar to strain CAI4 except when cells were incubated in the presence of 4 mM H2O2 for 120 min (P < 0.05). Western blot analysis was used to determine the role of Chk1p in phosphorylation of Hog1p under oxidative or osmotic stress. It was found that Hog1p was phosphorylated in the chk1 mutant similar to wild-type CAF2-1 cells, although the temporal events of phosphorylation differed slightly in mutant cells. These results show that transcription of CHK1, as measured by the lacZ reporter assay, is statistically increased when cells are exposed to several types of stress or when incubated in 10 % serum in a mutant-specific background and at a specific time point. Of importance, our data also suggest that lacZ expression is indirectly or directly regulated by the HOG1 MAP kinase pathway, although a determination of its position in this pathway or in a cross-talking pathway awaits additional studies.