Granule-bound starch synthase cDNA in Chlorella kessleri 11 h: cloning and regulation of expression by CO2 concentration

The cDNA for the granule-bound starch synthase (GBSS; ADP-glucose-starch glucosyltransferase, EC 2.4.1.21) of Chlorella kessleri 11 h was isolated and characterized. CkGBSS encodes a 609-amino acid polypeptide (65,627 Da) that includes an N-terminal hydrophobic signal peptide of 55 amino acids. The deduced amino acid sequence of the mature CkGBSS polypeptide shares a greater identity (65%) to that of the GBSS protein of Chlamydomonas reinhardtii, than to those of vascular plant species, but does not have the extra-long C-terminal sequence found in C. reinhardtii. When CO2 concentration was decreased from 3 to 0.04% (air level) in light, the levels of CkGBSS mRNA, CkGBSS protein, and GBSS activity increased. Under this condition, pyrenoid and pyrenoid starch developed, and the relative amount of amylose in starch increased. These observations suggest that low CO2 level up-regulates GBSS biosynthesis at the transcriptional level.