Mutants of Chlamydomonas with Aberrant Responses to Sulfur Deprivation.

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Abstract
In the absence of sulfur, Chlamydomonas reinhardtii, a unicellular green alga, increases its rate of sulfate import and synthesizes several periplasmic proteins, including an arylsulfatase (Ars). These changes appear to help cells acclimate to a sulfur-deficient environment. The elevated rate of sulfate import results from an increase in the capacity and affinity of the transport system for sulfate. The synthesis of Ars, a periplasmic enzyme that cleaves sulfate from aromatic compounds, enables cells to use these molecules as a source of sulfur when free sulfate is not available. To characterize the ways in which C. reinhardtii perceives changes in the sulfur status of the environment and regulates its responses to these changes, we mutagenized cells and isolated strains exhibiting aberrant accumulation of Ars activity. These mutants were characterized for Ars activity, ars mRNA accumulation, periplasmic protein accumulation, and sulfate transport activity when grown in both sulfur-sufficient and sulfur-deficient conditions. All of the mutants exhibited pleiotropic effects with respect to several of these responses. Strains harboring double mutant combinations were constructed and characterized for Ars activity and ars mRNA accumulation. From the mutant phenotypes, we inferred that both positive and negative regulatory elements were involved in the acclimation process. Both the epistatic relationships among the mutations and the effects of the lesions on the responses of C. reinhardtii to sulfur limitation distinguished these mutants from similar mutants in Neurospora crassa.