CARBON-DIOXIDE AND LIGHT REGULATION OF PROMOTERS CONTROLLING THE EXPRESSION OF MITOCHONDRIAL CARBONIC-ANHYDRASE IN CHLAMYDOMONAS-REINHARDTII

Nuclear genes coding for carbonic anhydrase, a major mitochondrial con stituent in Chlamydomonas reinhardtii grown under limited CO2, were ch aracterized. Two genes, ca1 and ca2, were found within 7 kb of genomic DNA, organized 'head to head' in a large inverted repeat. The DNA seq uences for the two genes were very similar, even in the promoter regio ns and in introns, indicating that the repeat is a result of a recent duplication. To study gene regulation, elements from the upstream regi on of cal were fused to the arylsulphatase reporter gene. After transf ormation,the expression of arylsulphatase was regulated similarly to t he endogenous ca1/ca2 genes, even when the promoter was trimmed down t o 194 nt. Expression could not be detected when 5% CO2 was bubbled int o the growth medium, but was induced within hours after transfer to ai r. The cal promoter was not induced in low light, but at intermediate light levels its activity was dependent on the irradiance. O-2 concent ration had no effect on the promoter activity, indicating that photore spiratory metabolites are not triggering the response. The availabilit y of cells transformed with a CO2-regulated reporter gene should facil itate further studies on the metabolic adaptations that occur in some green algae in response to the external CO2 level.