The conversion of r-carotene to astaxanthin is a complex pathway network, in which two steps of hydroxylation and two steps ofketolation are catalyzed by β-carotene hydroxylase （CrtZ） and β-carotene ketolase （Crt...The conversion of r-carotene to astaxanthin is a complex pathway network, in which two steps of hydroxylation and two steps ofketolation are catalyzed by β-carotene hydroxylase （CrtZ） and β-carotene ketolase （CrtW） respectively. Here, astaxanthin biosynthesis path- way was constructed in Saccharomyces cerevisiae by introducing heterologous CrtZ and CrtW into an existing high r-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 Scl 18060 with CrtW from Brevundimonas vesicu- laris 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_Scl 18060 was replaced from FBAlp to TEFlp, 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 crtWwere 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 timecourse, consequently leading to no significant improve- ment 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 lation by employing the key desired compounds accumu- enzyme（s） in microbes.展开更多
基金This work was supported by the International S＆T Cooperation Program of China （2015DFA00960）, the National Natural Science Foundation of China （Grant Nos. 31600052 and 21676192） and Innovative Talents and Platform Program of Tianjin （16PTSYJC00050）.
文摘The conversion of r-carotene to astaxanthin is a complex pathway network, in which two steps of hydroxylation and two steps ofketolation are catalyzed by β-carotene hydroxylase （CrtZ） and β-carotene ketolase （CrtW） respectively. Here, astaxanthin biosynthesis path- way was constructed in Saccharomyces cerevisiae by introducing heterologous CrtZ and CrtW into an existing high r-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 Scl 18060 with CrtW from Brevundimonas vesicu- laris 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_Scl 18060 was replaced from FBAlp to TEFlp, 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 crtWwere 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 timecourse, consequently leading to no significant improve- ment 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 lation by employing the key desired compounds accumu- enzyme（s） in microbes.
