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ABSTRACTThe Chlamydomonas reinhardtiitranscription factor PSR1 is required for the control of activities involved in scavenging phosphate from the environment during periods of phosphorus limitation. Increased scavenging activity reflects the development of high-affinity phosphate transport and the expression of extracellular phosphatases that can cleave phosphate from organic compounds in the environment. A comparison of gene expression patterns using microarray analyses and quantitative PCRs with wild-type and psr1mutant cells deprived of phosphorus has revealed that PSR1 also controls genes encoding proteins with potential “electron valve” functions—these proteins can serve as alternative electron acceptors that help prevent photodamage caused by overexcitation of the photosynthetic electron transport system. In accordance with this finding, phosphorus-starved psr1mutants die when subjected to elevated light intensities; at these intensities, the wild-type cells still exhibit rapid growth. Acclimation to phosphorus deprivation also involves a reduction in the levels of transcripts encoding proteins involved in photosynthesis and both cytoplasmic and chloroplast translation as well as an increase in the levels of transcripts encoding stress-associated chaperones and proteases. Surprisingly, phosphorus-deficient psr1cells (but not wild-type cells) also display expression patterns associated with specific responses to sulfur deprivation, suggesting a hitherto unsuspected link between the signal transduction pathways involved in controlling phosphorus and sulfur starvation responses. Together, these results demonstrate that PSR1 is critical for the survival of cells under conditions of suboptimal phosphorus availability and that it plays a key role in controlling both scavenging responses and the ability of the cell to manage excess absorbed excitation energy.

The Chlamydomonas reinhardtii transcription factor PSR1 is required for the control of activities involved in scavenging phosphate from the environment during periods of phosphorus limitation. Increased scavenging activity reflects the development of high-affinity phosphate transport and the expression of extracellular phosphatases that can cleave phosphate from organic compounds in the environment. A comparison of gene expression patterns using microarray analyses and quantitative PCRs with wild-type and psr1 mutant cells deprived of phosphorus has revealed that PSR1 also controls genes encoding proteins with potential "electron valve" functions—these proteins can serve as alternative electron acceptors that help prevent photodamage caused by overexcitation of the photosynthetic electron transport system. In accordance with this finding, phosphorus-starved psr1 mutants die when subjected to elevated light intensities; at these intensities, the wild-type cells still exhibit rapid growth. Acclimation to phosphorus deprivation also involves a reduction in the levels of transcripts encoding proteins involved in photosynthesis and both cytoplasmic and chloroplast translation as well as an increase in the levels of transcripts encoding stress-associated chaperones and proteases. Surprisingly, phosphorus-deficient psr1 cells (but not wild-type cells) also display expression patterns associated with specific responses to sulfur deprivation, suggesting a hitherto unsuspected link between the signal transduction pathways involved in controlling phosphorus and sulfur starvation responses. Together, these results demonstrate that PSR1 is critical for the survival of cells under conditions of suboptimal phosphorus availability and that it plays a key role in controlling both scavenging responses and the ability of the cell to manage excess absorbed excitation energy.

Abstract: During sulfur deprivation, the photosynthetic green alga Chlamydomonas reinhardtii develops a high-affinity sulfate uptake system and increases the expression of genes encoding proteins involved in sulfur assimilation. Although two regulatory elements, SAC1 and SAC3, have been shown to be required for normal acclimation of C. reinhardtii to sulfur deprivation, a number of other regulatory elements appear to also be involved. The molecular mechanisms by which these regulatory elements function are largely unknown. This manuscript presents our current knowledge of sulfur deprivation responses and the regulation of these responses in C. reinhardtii. In addition, we present preliminary results of a sub-saturation screen for novel sulfur acclimation mutants of C. reinhardtii. A speculative model, incorporating the activities of established regulatory elements with putative novel components of the signal transduction pathway(s) is discussed.

The unicellular green alga Chlamydomonas reinhardtii is a valuable model for studying metal metabolism in a photosynthetic background. A search of the Chlamydomonas expressed sequence tag database led to the identification of several components that form a copper-dependent iron assimilation pathway related to the high-affinity iron uptake pathway defined originally for Saccharomyces cerevisiae. They include a multicopper ferroxidase (encoded by Fox1), an iron permease (encoded by Ftr1), a copper chaperone (encoded byAtx1), and a copper-transporting ATPase. A cDNA, Fer1, encoding ferritin for iron storage also was identified. Expression analysis demonstrated that Fox1 and Ftrl were coordinately induced by iron deficiency, as were Atx1 and Fer1, although to lesser extents. In addition, Fox1 abundance was regulated at the posttranscriptional level by copper availability. Each component exhibited sequence relationship with its yeast, mammalian, or plant counterparts to various degrees; Atx1 of C. reinhardtii is also functionally related with respect to copper chaperone and antioxidant activities. Fox1 is most highly related to the mammalian homologues hephaestin and ceruloplasmin; its occurrence and pattern of expression in Chlamydomonas indicate, for the first time, a role for copper in iron assimilation in a photosynthetic species. Nevertheless, growth of C. reinhardtii under copper- and iron-limiting conditions showed that, unlike the situation in yeast and mammals, where copper deficiency results in a secondary iron deficiency, copper-deficient Chlamydomonas cells do not exhibit symptoms of iron deficiency. We propose the existence of a copper-independent iron assimilation pathway in this organism.

ABSTRACTThe unicellular green alga Chlamydomonas reinhardtiiis a valuable model for studying metal metabolism in a photosynthetic background. A search of the Chlamydomonasexpressed sequence tag database led to the identification of several components that form a copper-dependent iron assimilation pathway related to the high-affinity iron uptake pathway defined originally for Saccharomyces cerevisiae. They include a multicopper ferroxidase (encoded by Fox1), an iron permease (encoded by Ftr1), a copper chaperone (encoded by Atx1), and a copper-transporting ATPase. A cDNA, Fer1, encoding ferritin for iron storage also was identified. Expression analysis demonstrated that Fox1and Ftr1were coordinately induced by iron deficiency, as were Atx1and Fer1, although to lesser extents. In addition, Fox1 abundance was regulated at the posttranscriptional level by copper availability. Each component exhibited sequence relationship with its yeast, mammalian, or plant counterparts to various degrees; Atx1 of C. reinhardtiiis also functionally related with respect to copper chaperone and antioxidant activities. Fox1 is most highly related to the mammalian homologues hephaestin and ceruloplasmin; its occurrence and pattern of expression in Chlamydomonasindicate, for the first time, a role for copper in iron assimilation in a photosynthetic species. Nevertheless, growth of C. reinhardtiiunder copper- and iron-limiting conditions showed that, unlike the situation in yeast and mammals, where copper deficiency results in a secondary iron deficiency, copper-deficient Chlamydomonascells do not exhibit symptoms of iron deficiency. We propose the existence of a copper-independent iron assimilation pathway in this organism.

Chlamydomonas reinhardtii activates Cpx1, Cyc6, and Crd1, encoding, respectively, coproporphyrinogen oxidase, cytochrome c(6), and a novel di-iron enzyme when transferred to oxygen-deficient growth conditions. This response is physiologically relevant because C. reinhardtii experiences these growth conditions routinely, and furthermore, one of the target genes, Crd1, is functionally required for normal growth under oxygen-depleted conditions. The same genes are activated also in response to copper-deficiency through copper-response elements that function as target sites for a transcriptional activator. The core of the copper-response element, GTAC, is required also for the hypoxic response, as is a trans-acting locus, CRR1. Mercuric ions, which antagonize the copper-deficiency response, also antagonize the oxygen-deficiency response of these target genes. Taken together, these observations suggest that the oxygen- and copper-deficiency responses share signal transduction components. Nevertheless, whereas the copper-response element is sufficient for the nutritional copper response, the oxygen-deficiency response requires, in addition, a second cis-element, indicating that the response to oxygen depletion is not identical to the nutritional copper response. The distinction between the two responses is also supported by comparative analysis of the response of the target genes, Cyc6, Cpx1, and Crd1, to copper versus oxygen deficiency. A Crr1-independent pathway for Hyd1 expression in oxygen-depleted C. reinhardtii demonstrates the existence of multiple oxygen/redox-responsive circuits in this model organism.