Targeting of exogenous β-carotene oxygenase into the chloroplast is essential for its efficient function in the microalga Chlamydomonas reinhardtii

Carotenoids synthesis takes place in the chloroplast but is catalysed by enzymes encoded by nuclear genes. These enzymes are thus synthesised in the cytoplasm as precursor polypeptides with an amino-terminal extension, the transit peptide (tp), and targeted to their final location in the chloroplast. beta-carotene oxygenase (BKT) mediates the additional of keto groups to the position C4 of beta-ione-rings of carotenoids. We studied the effect of the transit peptide sequence of Rubisco small subunit (RbcS2), Ferredoxine (Fd) or Phytoene desaturase (Pds) from Chlamydomonas corresponding proteins and the absence of tp on the functionality of exogenous bktl in transgenic Chlamydomonas reinhardtii. RbcS, Fd and Pds are targeted to the chloroplast stroma, intertylakoid lumen and tylakoid membrane respectively. Our results indicate that the presence of a chloroplastic transit peptide is essential for the efficient production of ketocarotenoids in transgenic strains of the microalga Chlamydomonas reinhardtii.