S is an essential element present in proteins, lipids, and various metabolites

Sulfur Deprivation

Sulfur is an essential element present in proteins, lipids, and various metabolites. It is also critical for the association of metal molecules with proteins, photoprotection and signal transduction (Grossman and Takahashi, 2001). Sulfur can be limiting in the environment and strongly influence ecosystem composition. It may also limit plant productivity in certain agricultural settings . Sulfur limitation results in reduced quality and yields of seeds and, upon severe limitation, the growth of the plant may be stunted. In recent history many soils have accumulated high sulfur levels, which is a result of either the administration of fertilizers contaminated with SO₄^2- salts or exposure to pollutants present in acid rain. However, with increased fertilizer purity and decreased occurrence of acid rains, low levels of available sulfur in diverse ecosystems can limit the growth and development of vascular plants.

We have used C. reinhardtii to study sulfur stress responses and to isolated mutants in these responses (Davies et al., 1998; Yildiz et al., 1994, 1996; Takahashi et al., 2001). The responses observed during sulfur stress include increased activity of arylsulfatase and sulfate transport, cessation of cell growth and division and a decline in most metabolic processes in the cell, including photosynthetic oxygen evolution (which shows a marked decline). Several C. reinhardtii mutant have been isolated that cannot properly respond to sulfur deprivation conditions (Davies et al., 1994). One of these mutants, sac1, is markedly defective in its ability to acclimate to sulfur limitation conditions (Davies et al., 1996). Another mutant, sac3, is aberrant for a serine-threonine protein kinase and shows constitutive expression of arylsulfatase activit (Davies et al., 1999). Indeed, the mutant cells dies much more rapidly than wild-type cells during sulfur deprivation; this is shown in Figure 1. As also shown in Figure 1, the cells can be saved by placing them in the dark or feeding them DCMU (which blocks photosynthetic electron flow) after placing them under sulfur starvation conditions. These results strongly suggest that continued photosynthetic electron flow is killing the cells during sulfur stress. Furthermore, many transcripts that are either positively or negatively regulated in wild-type cells during S deprivation are not properly regulated in the sac1 mutant (although others are). Interestingly, lesions at 4 different loci on the C. reinhardtii genome exhibit a sac1 phenotype, which includes more rapid death than wild-type cells and an inability to synthesize arylsulfatase during S deprivation. Our goals for this project are to study the role that the regulatory protein Sac1 plays in the acclimation of C. reinhardtii to S availability in the environment and in a more general sense to improve our understanding of how photosynthetic organisms acclimate to a dynamic environment. We are beginning to take a more biochemical approach to identify the interactions of Sac1 with other proteins in the cell.

Figure 1. The death of sac1 mutant in during sulfur deprivation and rescue from death by placing the cultures in the dark or feeding them DCMU.

References

Davies, J. D. and A. R. Grossman (1998) Responses to deficiencies in macronutrients. In The Molecular Biology of Chlamydomonas. (Ed. J.-D. Rochaix, M. Goldschmidt-Clermont and S. Merchant). Kluwer Publishers, Dordrecht. Pp 613-635.

Davies, J. F. Yildiz, and A. R. Grossman (1994) Mutants of Chlamydomonas reinhardtii with aberrant responses to sulfur deprivation. Plant Cell 6:53-63.

Davies, J. D., F. Yildiz and A. R. Grossman (1999) The involvement of an SNF1-like serine-threonine kinase in the acclimation of Chlamydomonas reinhardtii to sulfur limitation. Plant Cell 11:1179-1190.

Davies, J., F. Yildiz and A. R. Grossman, (1996) Sac1, a putative regulator that is critical of survival of Chlamydomonas reinhardtii during sulfur deprivation. EMBO J 15:2150-2159.

Grossman, A. R. (2001) Inorganic nutrient control of gene expression in photosynthetic eukaryotes. Annu Rev Plant Physiol Plant Mol Biol, 52:163-210.

Takahashi, H., C. Braby and A.R. Grossman (2001) Specific sulfur-stress induced genes encoding putative cell wall proteins in Chlamydomonas reinhardtii. Plant Physiol 127:665-673.

Yildiz, F., J. Davies and A. R. Grossman (1996) Controlled expression of the ATP sulfurylase gene of Chlamydomonas reinhardtii. Plant Physiol 112:669-675.

Yildiz, F., J. P. Davies, and A. R. Grossman. 1994. Characterization of a high affinity sulfate transport system that accumulates during sulfur-limited growth of C. reinhardtii. Plant Physiol 104:981-987.