Regulation of the expression of photorespiratory genes of Chlamydomonas reinhardtii

The regulation of the photorespiratory pathway of Chlamydomonas reinhardtii during a shift from high to low CO2 conditions was investigated. To this end, a set of C. reinhardtii cDNA sequences for known photorespiratory enzymes was assembled using the C. reinhardtii EST data and primary sequencing data. The cDNA sequences of the proposed photorespiratory genes are presented. A subset of these genes was used for expression analyses under varied conditions. Expression data indicates that there was a rapid and coordinated induction of photorespiratory and carbon dioxide concentrating mechanism (CCM) gene expression during a time course switch from elevated CO2 conditions (5% [v/v]) to low CO2 conditions (0.038% [v/v]). While the expression of photorespiratory and CCM genes was coordinated during the initial change in CO2 level, the response of these two sets of genes to the CO2 level was not identical. Unlike the sustained high levels of CCM mRNAs seen under low CO2 conditions, photorespiratory mRNAs showed a transient increase in abundance in time course experiments. In addition, the expression of these photorespiratory genes was reduced in cia5, a C. reinhardtii strain that lacks a transcription factor required for the induction of genes involved in the CCM. From these observations, there appears to be coordination in the expression of the genes involved in the delivery of CO2 to Rubisco and the genes involved in the metabolism of the photorespiratory products that form when the CO2 level is low. Further expression analyses were carried out to gain a better understanding of the signal that induces CCM and photorespiration of C. reinhardtii. Low CO2 concentration appears to be the only signal for the induction of photorespiration and CCM under the tested conditions