Regulation of ß-carotene hydroxylase in flagellates of the unicellular green alga Haematococcus pluvialis

Abstract

The volvocal green alga Haematococcus pluvialis Flotow is known for its massive accumulation of ketocarotenoids under various stress conditions. Astaxanthin makes up the main part of the secondary carotenoids and is accumulated mostly in an esterified form in extraplastidic lipid vesicles. Recently, we have studied the cellular localization and regulation of two enzymes involved in the synthesis of secondary carotenoids, namely, phytoene desaturase and ß-carotene oxygenase (Grünewald et al., 2000, Plant Physiol. 122, 1261-1268; Grünewald et al., 2001, J. Biol. Chem., 276, 6023-6029). Here, we studied the enzyme ß-carotene hydroxylase in more detail. A gene for ß-carotene hydroxylase had been cloned in H. pluvialis (Linden, 1999, BBA, 1446, 203-212). Because no isoforms were found at the gene level, the enzyme catalyzing the introduction of hydroxy functions at position C3 of the ß-ionone ring is assumed to have a dual function in H. pluvialis being responsible for the formation of the photosynthetic (and therefore primary) carotenoid zeaxanthin from ß-carotene and of the secondary carotenoid astaxanthin from canthaxanthin. We report on mRNA and protein levels of the enzyme studied by northern blot and western blot analysis and discuss the results in terms of regulation and compartmentation.