And 1.0 M KCL) [24]. The observations provided an interesting possibility that additional inputs into Pbs2 may exist [24,25]. To identify the alternative pathway, we constructed the double mutant ssk1Dste11D, and the triple mutant ste11Dssk2Dssk22D. We carried out the phosphorylation level of Hog1p in the mutant CASIN ssk1Dste11D and ste11Dssk2Dssk22D under a wide range of osmotic stress conditions (NaCl, KCl and sorbitol, from 0.2 M to 1.0 M). The results, including also measurements on the wide type strain, are shown in Figure 1. We observed that the Hog1p was activated in the ssk1Dste11D mutant at 0.6 M sorbitol or a higher concentration (Figure 1A). However, Hog1p phosphorylation was not detected under mild osmotic stress (0.2 M and 0.4 M sorbitol/NaCl) in the double mutant (Figure 1A and 1D). In contrast, the phosphorylation of Hog1p could not be detected in the ste11Dssk2Dssk22D mutant in the wide range concentration of osmotic stress (NaCl, KCl and sorbitol, from 0.2 M to 1.0 M) (Figure 1 C). Under severe osmotic shock, for instance, 1.0 M sorbitol/NaCl, the phosphorylation of Hog1p peaked within 10 min and lasted for more than 60 min in the wild type strain (Figures 1B and 1E). In the ssk1Dste11D mutant, although the level of phosphorylation of Hog1p reached was high, the duration was short. In the ssk1Dste11D mutant, the phosphorylation of Hog1p disappeared within 20 min under 1.0 M sorbitol (Figure 1A). This result is consistent with the transcriptional profiles of A-196 osmoregulated genes in the strain ssk1Dste11D [24]. The expression of several osmoregulated genes (STL1, GRE2) in ssk1Dste11D was induced at high level under 0.5 M KCl but the duration of the induction was shorter than that of the wide type strain [24]. Besides, the strain ssk1Dste11D exhibited much better growth than the hog1D mutant and the ste11Dssk2Dssk22D mutant under osmotic stress (Figure 1F). However, the growth of ssk1Dste11D mutant under osmotic stress depended greatly on the type of osmostressor. The mutant ssk1Dste11D show better osmoresistance under nonionic osmostressor (sorbitol) (Figure 1 F) than under ionic stress even the Hog1p was similarly phosphorylated under ionic stress. The ssk1Dste11D cells grew better under KCL stress than under NaCL stress (Figure 1 F).also been reported that the ssk1Dssk22Dsho1D cells showed better resistance to 500 mM NaCl and 1.5 M sorbitol than ssk1D ssk2Dssk22Dsho1D cells did [26]. To further analyze the alternate activation pathway independent of Ssk1p and Ste11p, we constructed two triple mutants: the ste11Dssk1Dssk2D mutant and ste11Dssk1Dssk22D mutant to analyze the phosphorylation state of Hog1p under osmotic stress. Figure 2 shows measurements of the phosphorylation level of Hog1p as well the growth phenotypes in our experiments with the mutant cells. The HOG pathway was activated in the absence of Ste11p, Ssk1p and Ssk22p (Figure 2A) and was inactive if the STE11, SSK1 and SSK2 were deleted (Figure 2B). The Hog1p was significantly phosphorylated in the ste11Dssk1Dssk22D mutant under severe osmotic stress (higher than 0.6 M sorbitol). This implies that the MAPKKK Ssk2p can be activated in the absence of Ssk1p under severe osmotic stress. Moderate osmotic stress (concentration lower than 0.4 M sorbitol), on the other hand, could not lead to significant phosphorylation of Hog1p. The phosphorylation pattern of Hog1p under the stress in ste11Dssk1Dssk22D mutant in Figure 2A is similar to that of the ssk1D ste11D mutant s.And 1.0 M KCL) [24]. The observations provided an interesting possibility that additional inputs into Pbs2 may exist [24,25]. To identify the alternative pathway, we constructed the double mutant ssk1Dste11D, and the triple mutant ste11Dssk2Dssk22D. We carried out the phosphorylation level of Hog1p in the mutant ssk1Dste11D and ste11Dssk2Dssk22D under a wide range of osmotic stress conditions (NaCl, KCl and sorbitol, from 0.2 M to 1.0 M). The results, including also measurements on the wide type strain, are shown in Figure 1. We observed that the Hog1p was activated in the ssk1Dste11D mutant at 0.6 M sorbitol or a higher concentration (Figure 1A). However, Hog1p phosphorylation was not detected under mild osmotic stress (0.2 M and 0.4 M sorbitol/NaCl) in the double mutant (Figure 1A and 1D). In contrast, the phosphorylation of Hog1p could not be detected in the ste11Dssk2Dssk22D mutant in the wide range concentration of osmotic stress (NaCl, KCl and sorbitol, from 0.2 M to 1.0 M) (Figure 1 C). Under severe osmotic shock, for instance, 1.0 M sorbitol/NaCl, the phosphorylation of Hog1p peaked within 10 min and lasted for more than 60 min in the wild type strain (Figures 1B and 1E). In the ssk1Dste11D mutant, although the level of phosphorylation of Hog1p reached was high, the duration was short. In the ssk1Dste11D mutant, the phosphorylation of Hog1p disappeared within 20 min under 1.0 M sorbitol (Figure 1A). This result is consistent with the transcriptional profiles of osmoregulated genes in the strain ssk1Dste11D [24]. The expression of several osmoregulated genes (STL1, GRE2) in ssk1Dste11D was induced at high level under 0.5 M KCl but the duration of the induction was shorter than that of the wide type strain [24]. Besides, the strain ssk1Dste11D exhibited much better growth than the hog1D mutant and the ste11Dssk2Dssk22D mutant under osmotic stress (Figure 1F). However, the growth of ssk1Dste11D mutant under osmotic stress depended greatly on the type of osmostressor. The mutant ssk1Dste11D show better osmoresistance under nonionic osmostressor (sorbitol) (Figure 1 F) than under ionic stress even the Hog1p was similarly phosphorylated under ionic stress. The ssk1Dste11D cells grew better under KCL stress than under NaCL stress (Figure 1 F).also been reported that the ssk1Dssk22Dsho1D cells showed better resistance to 500 mM NaCl and 1.5 M sorbitol than ssk1D ssk2Dssk22Dsho1D cells did [26]. To further analyze the alternate activation pathway independent of Ssk1p and Ste11p, we constructed two triple mutants: the ste11Dssk1Dssk2D mutant and ste11Dssk1Dssk22D mutant to analyze the phosphorylation state of Hog1p under osmotic stress. Figure 2 shows measurements of the phosphorylation level of Hog1p as well the growth phenotypes in our experiments with the mutant cells. The HOG pathway was activated in the absence of Ste11p, Ssk1p and Ssk22p (Figure 2A) and was inactive if the STE11, SSK1 and SSK2 were deleted (Figure 2B). The Hog1p was significantly phosphorylated in the ste11Dssk1Dssk22D mutant under severe osmotic stress (higher than 0.6 M sorbitol). This implies that the MAPKKK Ssk2p can be activated in the absence of Ssk1p under severe osmotic stress. Moderate osmotic stress (concentration lower than 0.4 M sorbitol), on the other hand, could not lead to significant phosphorylation of Hog1p. The phosphorylation pattern of Hog1p under the stress in ste11Dssk1Dssk22D mutant in Figure 2A is similar to that of the ssk1D ste11D mutant s.
