Functional Role and Mutational Analysis of the Phytoene Synthase from the Halophilic Euryarchaeon Haloferax volcanii in Bacterioruberin Biosynthesis

Phytoene synthase (PSY) is one of key enzymes in carotenogenesis that catalyze two molecules of geranylgeranyl diphosphate to produce phytoene. PSY is widespread in bacteria, archaea, and eukaryotes. Currently, functional role and catalytic mechanism of archaeal PSY homologues have not been fully clarified due to the limited reports. Herein, we identified a rate-limiting role of the PSY from the halophilic euryarchaeon Haloferax volcanii (HVO-PSY) in bacterioruberin biosynthesis and dissected the roles of its seven conserved residues. Compared with the parental H. volcanii strain, the recombinant HVO-PSY strain had the increased bacterioruberin yields, confirming that HVO-PSY is a rate-limiting enzyme in bacterioruberin biosynthesis in H. volcanii. Additionally, we optimized the culture condition for bacterioruberin biosynthesis in the recombinant strain: 150 g/L NaCl, 45 °C, and pH 6.5-7.5. Mutational data demonstrate that residues D47, D51, D110, D168, D172, and R178 of HVO-PSY are essential for catalysis since these recombinant mutant strains harboring these mutations in the enzyme had the reduced bacterioruberin yields relative to the recombinant wild-type strain. Overall, our findings have verified the rate-limiting role of HVO-PSY in bacterioruberin biosynthesis in H. volcanii and clarified the essential roles of its conserved residues D47, D51, D110, D168, D172, and R178 in catalysis.