Engineering of β-carotene hydroxylase and ketolase for astaxanthin overproduction in Saccharomyces cerevisiae

The conversion of beta-carotene to astaxanthin is a complex pathway network, in which two steps of hydroxylation and two steps of ketolation are catalyzed by beta-carotene hydroxylase (CrtZ) and beta-carotene ketolase (CrtW) respectively. Here, astaxanthin biosynthesis pathway was constructed in Saccharomyces cerevisiae by introducing heterologous CrtZ and CrtW into an existing high beta-carotene producing strain. Both genes crtZ and crtW were codon optimized and expressed under the control of constitutive promoters. Through combinatorial expression of CrtZ and CrtW from diverse species, nine strains in dark red were visually chosen from thirty combinations. In all the selected strains, strain SyBE_Sc118060 with CrtW from Brevundimonas vesicularis DC263 and CrtZ from Alcaligenes sp. strain PC-1 achieved the highest astaxanthin yield of 3.1 mg/g DCW. Protein phylogenetic analysis shows that the shorter evolutionary distance of CrtW is, the higher astaxanthin titer is. Further, when the promoter of crtZ in strain SyBE_Sc118060 was replaced from FBA1p to TEF1p, the astaxanthin yield was increased by 30.4% (from 3.4 to 4.5 mg/g DCW). In the meanwhile, 33.5-fold increase on crtZ transcription level and 39.1-fold enhancement on the transcriptional ratio of crtZ to crtW were observed at early exponential phase in medium with 4% (w/v) glucose. Otherwise, although the ratio of crtZ to crtW were increased at mid-, late-exponential phases in medium with 2% (w/v) glucose, the transcription level of both crtZ and crtW were actually decreased during the whole time course, consequently leading to no significant improvement on astaxanthin production. Finally, through high cell density fed-batch fermentation using a carbon source restriction strategy, the production of astaxanthin in a 5-L bioreactor reached to 81.0 mg/L, which was the highest astaxanthin titer reported in yeast. This study provides a reference to greatly enhance desired compounds accumulation by employing the key enzyme(s) in microbes.