The green alga Chlamydomonas reinhardtii does not synthesize high‐value ketocarotenoids like canthaxanthin and astaxanthin; however, a β‐carotene ketolase (CrBKT) can be found in its genome. CrBKT is poorly expressed, contains a long C‐terminal extension not found in homologues and likely represents a pseudogene in this alga. Here, we used synthetic redesign of this gene to enable its constitutive overexpression from the nuclear genome of C. reinhardtii. Overexpression of the optimized CrBKT extended native carotenoid biosynthesis to generate ketocarotenoids in the algal host causing noticeable changes the green algal colour to reddish‐brown. We found that up to 50% of native carotenoids could be converted into astaxanthin and more than 70% into other ketocarotenoids by robust CrBKT overexpression. Modification of the carotenoid metabolism did not impair growth or biomass productivity of C. reinhardtii, even at high light intensities. Under different growth conditions, the best performing CrBKT overexpression strain was found to reach ketocarotenoid productivities up to 4.3 mg/L/day. Astaxanthin productivity in engineered C. reinhardtii shown here might be competitive with that reported for Haematococcus lacustris (formerly pluvialis) which is currently the main organism cultivated for industrial astaxanthin production. In addition, the extractability and bio‐accessibility of these pigments were much higher in cell wall‐deficient C. reinhardtii than the resting cysts of H. lacustris. Engineered C. reinhardtii strains could thus be a promising alternative to natural astaxanthin producing algal strains and may open the possibility of other tailor‐made pigments from this host.

中文翻译：

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将绿藻变红：通过莱茵衣藻基因内假基因复兴工程化虾青素生物合成

绿藻莱茵衣藻不合成高价值的酮类胡萝卜素，如角黄素和虾青素；然而，在其基因组中可以发现β-胡萝卜素酮酶（Cr BKT）。Cr BKT 表达较差，包含同源物中未发现的长 C 末端延伸，并且可能代表该藻类中的假基因。在这里，我们对该基因进行了合成重新设计，使其能够从莱茵衣藻的核基因组中组成型过度表达。优化的Cr BKT 的过度表达延长了天然类胡萝卜素的生物合成，在藻类宿主中生成酮类胡萝卜素，导致绿藻颜色显着变为红棕色。我们发现，通过强劲的Cr BKT 过表达，高达 50% 的天然类胡萝卜素可转化为虾青素，超过 70% 的天然类胡萝卜素可转化为其他酮类胡萝卜素。即使在高光强度下，类胡萝卜素代谢的改变也不会损害莱茵衣藻的生长或生物量生产力。在不同的生长条件下，发现表现最好的Cr BKT 过表达菌株的酮类胡萝卜素生产率高达 4.3 mg/L/天。这里显示的工程化莱茵藻的虾青素生产力可能与报道的湖红球藻（以前称为雨生红球藻）的生产力具有竞争力，后者是目前为工业虾青素生产而培养的主要生物体。此外，在细胞壁缺陷的莱茵衣藻中，这些色素的可提取性和生物可及性比湖莱氏藻的休眠包囊中的要高得多。因此，工程化的莱茵衣藻菌株可能成为天然虾青素产生藻类菌株的有前途的替代品，并可能开启从此宿主生产其他定制色素的可能性。