Chlorophyll a Synthesis in Rhodobacter sphaeroides by Chlorophyll Synthase of Nicotiana tabacum

SIMPLE SUMMARY: Photosynthetic (PS) organisms utilize light energy via their PS apparatuses which contain PS pigments. Since a PS organism usually has limited wavelength regions of light absorption determined by the property of their pigments, introducing heterologous pigments can expand the light absorption regions, which could enhance the efficiency of light utilization. Rhodobacter sphaeroides is a bacteriochlorophyll a (BChl a)-containing PS bacterium which has been widely used in photosynthesis research and biotechnological applications. In this study, we attempted to introduce chlorophyll synthase (ChlG) into R. sphaeroides to synthesize chlorophyll a (Chl a) under BChl a formation. It was challenging because BChl a and its precursors can inhibit ChlG. However, ChlGs of angiosperm plants were found to possess resistance against these inhibitions. Thus, we selected ChlG of Nicotiana tabacum, which had the highest resistance among angiosperm ChlGs and achieved the formation of Chl a in the presence of BChl a in R. sphaeroides by expression of the enzyme. ABSTRACT: The production of phytylated chlorophyll a (Chl a(P)) in Rhodobacter sphaeroides, which uses phytylated bacteriochlorophyll a (BChl a(P)), is the first step in expanding the light absorption spectra. Unlike the chlorophyll synthase (ChlG) of the Synechocystis sp. PCC6803, ChlGs of angiosperms, including Arabidopsis thaliana, Nicotiana tabacum, Avena sativa, and Oryza sativa, showed bacteriochlorophyll synthase activity and resistance to inhibition by bacteriochlorophyllide a (BChlide a), geranylgeranylated BChl a (BChl a(GG)), and BChl a(P), collectively called bacteriochlorins. Among the angiosperm ChlGs, N. tabacum ChlG had the highest bacteriochlorophyll synthase activity and resistance to inhibition by bacteriochlorins. Expression of N. tabacum chlG in R. sphaeroides resulted in the formation of free Chl a(P) in the presence of BChl a(P) during photoheterotrophic growth, even though reactive oxygen species were generated.