Circadian expression of the carbonic anhydrase gene, Cah1, in Chlamydomonas reinhardtii.

We have investigated whether the expression of carbonic anhydrase genes (Cah1 and Cah2) is regulated by a circadian clock in Chlamydomonas. When cells were grown in ordinary air under 12 h light/12 h dark (LD) cycles, the levels of the Cah2 mRNA hardly altered during the cycles, while the Cah1 mRNA showed a strong diurnal rhythm. The rhythm of about 24 h continued at least 3 days even under continuous light. Temperature compensation of the rhythm was demonstrated, using cultures maintained at 16, 22, and 28 degrees C. These results indicate that the abundance of the Cah1 transcript is controlled by a circadian clock.