Rubisco activase transcript (rca) abundance increases when the marine unicellular green alga Chlorococcum littorale is grown under high-CO2 stress

Abstract

cDNA and the corresponding genomic DNA region encoding Rubisco activase were isolated from the unicellular green alga Chlorococcum littorale. The deduced amino acid sequence encoded by the cDNA was 403 amino acids long and exhibited important homology with those of other known Rubisco activases. Its N-terminal sequence was similar to the chloroplastic transit peptides in Chlamydomonas reinhardtii. The mature protein had a predicted molecular mass of 42 kDa. Five introns were located inside the genomic gene encoding Rubisco activase (rca). Genomic Southern blots indicated that two copies of the rca gene were present in the genome of C. littorale. The level of rca messenger RNA increased when cells of C. littorale were subjected to high-CO₂ stress (i.e. grown under at least 20% CO₂). Hsp70 heat-shock protein was also induced under high-CO₂ conditions and, as expected, was also induced at 35 °C. The rca gene, in contrast, was not induced at 35 °C, indicating that this gene was induced in response to the high CO₂ concentration and not to general stress. A search of the promoter-binding proteins by a gel retardation assay showed that, under the high-CO₂ conditions, a protein(s) which was probably an activator of the rca transcription was synthesized.