Chlamydomonas reinhardtii adapts to the stress of CO2-limiting conditions through the induction of a set of genes including CAH1, which encodes a periplasmic carbonic anhydrase. CAH1 is up-regulated under low-CO2 conditions (air containing 0.04% [v/v] CO2) in the presence of light, whereas it is down-regulated under high-CO2 conditions (5% [v/v] CO2) or in the dark. In an effort to identify cis-elements involved in the transcriptional regulation of CAH1, a series of 5'-nested deletions of the region upstream of CAH1 were fused to a promoterless arylsulfatase reporter gene (ARS). The upstream region from -651 to +41 relative to the transcription start site was sufficient to regulate the expression of ARS with kinetics similar to those of endogenous CAH1. Deletion of the region between -651 and -294 resulted in a significant decrease in the level of arylsulfatase activity expressed under low-CO2 conditions. The 543-bp upstream region from -651 to -109, without any promoter elements, CAAT-box, or TATA-box, could confer CO2 and light responsiveness on the beta(2)-tubulin minimal promoter. This 543-bp region was divided into two parts: a 358-bp silencer region from -651 to -294, which represses the minimal promoter activity under high-CO2 conditions, and a 185-bp enhancer region from -293 to -109, which activates the promoter under low-CO2 conditions in the presence of light.