Regulation of the expression of photorespiratory genes in Chlamydomonas reinhardtii

The regulation of the photorespiratory pathway in Chlamydomonas reinhardtii Dangeard 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 Chlamydomonas expressed sequence tag database and primary sequencing data. Expression data indicates that there is a rapid and coordinated induction of photorespiratory and CCM gene expression during a time course switch from high-CO2 conditions (5% (v/v)) to low-CO2 conditions (0.038% (v/v)). While the expression of photorespiratory and CCM genes are coordinated during the initial change in CO2 level, the response of these two sets of genes to the CO2 level is not identical. Unlike the sustained high levels of CCM mRNAs seen under low CO2 conditions, photorespiratory mRNAs show a transient increase in abundance in time course experiments. In addition, the expression of these photorespiratory genes is reduced in cia5, a C. reinhardtii strain that lacks a transcription factor required for the induction of genes involved in the CO2-concentrating mechanism. 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.