| Zinc is a required cofactor in proteins involved in respiration, transcription, and photosynthesis. But, this metal nutrient also poses a threat to cells if it is present in excess concentrations. Thus, zinc homeostasis must be maintained and regulated to ensure that essential proteins have enough metal to function while toxicity is avoided, even in the face of changing environmental conditions. Here, two strategies were used to identify zinc-responsive genes that may be involved in regulating zinc homeostasis in Chlamydomonas. In a targeted bioinformatic approach, homologs of known zinc-responsive genes from other organisms were used to identify 11 putative zinc transporters in Chlamydomonas. These genes were analyzed for zinc-responsive transcriptional regulation using real-time PCR. Two genes, ZRT1 and ZRT3, were found to be upregulated under both zinc-deficient and "zinc-shock" conditions, consistent with the model that these genes participate in a proactive system involved in preventing toxicity. To identify additional zinc-responsive genes with no previous assumptions based on gene identity, a genomic approach was initiated. RNA from Chlamydomonas cultures growing in zinc-replete and zinc-limited cultures was collected, and patterns of gene expression for the two conditions will be compared using newly developed Solexa tag sequencing technology, which directly quantifies whole-cell mRNA transcript profiles.
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