Glucose sensing and the mitochondrial alternative pathway are involved in the regulation of astaxanthin biosynthesis in the dark-grown Chlorella zofingiensis (Chlorophyceae).

The biosynthesis of the ketocarotenoid astaxanthin is a subject of scientific and industrial interest. The unicellular green alga Chlorella zofingiensis Dönz is able to grow and accumulate astaxanthin in the dark with exogenous glucose as sole carbon and energy source. In this study, the transcription of beta-carotenoid ketolase (BKT) and beta-carotenoid hydroxylase (CHYb) genes were surveyed to reveal the regulation of astaxanthin biosynthesis in dark-grown Chlorella zofingiensis. Coupled with glucose analogs and the hexokinase inhibitor glucosamine, we found that phosphorylation of glucose (glucose sensing) was essential to the increased transcription of BKT and CHYb genes and the accumulation of astaxanthin in the dark-grown cells. However, phosphorylation of glucose per se only up-regulated the transcription of CHYb and stimulated the synthesis of zeaxanthin. Blockage of the mitochondrial alternative pathway eliminated the glucose effects on the increased transcription of BKT and astaxanthin accumulation, suggesting that signals from alternative pathway was involved in the up-regulation of BKT transcription. In addition, citrate was shown to up-regulate the transcription of BKT independent of reactive oxygen species formation. Taken altogether, we conclude that in dark-grown Chlorella zofingiensis, the transcription of BKT and CHYb genes are differently regulated by the metabolism of glucose, through which the biosynthesis of astaxanthin is regulated.