A mutant of Chlamydomonas reinhardtii that cannot acclimate to low CO2 conditions has an insertion in the Hdh1 gene

Photosynthetic microorganisms must acclimate to environmental conditions, such as low CO2 environments or high light intensities, which may lead to photo- oxidative stress. In an effort to understand how photosynthetic microorganisms acclimate to these conditions, Chlamydomonas reinhardtii was transformed using the Ble(R) cassette, selected for Zeocin resistance and screened for colonies that showed poor growth at low CO2 levels. One of the insertional mutants obtained, named slc- 230, was shown to have a Ble(R) insert in the first exon of Hdh1, a novel, single copy gene. The predicted Hdh1 gene product has similarity to bacterial haloacid dehalogenase-like proteins, a protein family that includes phosphatases and epoxide hydrolases. In addition, Hdh1 is predicted to be localised to the chloroplast or mitochondria in C. reinhardtii. It was found that a genomic copy of wild type Hdh1 can complement slc- 230. Physiological studies were conducted to determine the effects of the altered expression of Hdh1 in slc- 230. slc- 230 grows slowly autotrophically in low CO2, exhibits a lower affinity for inorganic carbon, a decreasing photosynthetic rate over time and a lower content of chlorophylls and quenching xanthophylls than wild type cells. Some possible roles of Hdh1 in the acclimation to low CO2 conditions are discussed.