Peter Rubinelli, Surasak Suripornadulsil, and Richard T. Sayre

Department of Plant Biology, Ohio State University, Columbus, OH 43210

In this study we expand on previous work done to investigate the heavy metal binding capacity of the wild type Chlamydomonas reinhardtii cell wall. These studies indicate a role for glycoproteins of the Chlamydomonas cell wall in binding heavy metals and providing resistance to the toxic effects of these metals on the cell. Other studies using heavy metal sensitive mutants of fission yeast have shed light on intracellular detoxification mechanisms, as have biochemical studies of metallothioneins. In contrast to these studies, relatively little has been done to assess immediate or early changes in gene expression resulting from heavy metal exposure using gene expression screen technology. Using the method of mRNA differential display and a cell-wall-deficient mutant strain, we have identified several mRNA transcripts of Chlamydomonas reinhardtii that are induced by a brief (2 hour) exposure to 25 micromolar cadmium chloride. Sequence analysis of these transcripts indicates that they represent genes previously unknown to play a role in the response to heavy metal stress. These sequence analyses will be presented. One of these transcripts encodes a novel protein (designated H43) whose expression was previously found to be induced by very high concentrations of carbon dioxide. The H43 derived amino acid sequence contains an N-terminal signal peptide and several potential N-myristoylation and N-glycosylation sites, as well as a possible ATP/GTP binding motif. A homologous gene, HCR1, from Chlorococcum littorale, a marine green microalga, is induced by iron deficiency in addition to high carbon dioxide concentration, suggesting a possible role for HCR1 in iron uptake. We hypothesize that exposure to cadmium results in competition for uptake between cadmium and iron through one or more iron importers, resulting in intracellular sensing of iron depletion. The H43 gene likely represents part of the response to this iron depletion.