Engineered biosynthesis of bacteriochlorophyll g_F in Rhodobacter sphaeroides

Engineering photosynthetic bacteria to utilize a heterologous reaction center that contains a different (bacterio) chlorophyll could improve solar energy conversion efficiency by allowing cells to absorb a broader range of the solar spectrum. One promising candidate is the homodimeric type I reaction center from Heliobacterium modesticaldum. It is the simplest known reaction center and uses bacteriochlorophyll (BChl) g, which absorbs in the near-infrared region of the spectrum. Like the more common BChls a and b, BChl g is a true bacteriochlorin. It carries characteristic C3-vinyl and C8-ethylidene groups, the latter shared with BChl b. The purple phototrophic bacterium Rhodobacter (Rba.) sphaeroides was chosen as the platform into which the engineered production of BChl g_F, where F is farnesyl, was attempted. Using a strain of Rba. sphaeroides that produces BChl b_P, where P is phytyl, rather than the native BChl a_P, we deleted bchF, a gene that encodes an enzyme responsible for the hydration of the C3-vinyl group of a precursor of BChls. This led to the production of BChl g_P. Next, the crtE gene was deleted, thereby producing BChl g carrying a THF (tetrahydrofarnesol) moiety. Additionally, the bchG^Rs gene from Rba. sphaeroides was replaced with bchG^Hm from Hba. modesticaldum. To prevent reduction of the tail, bchP was deleted, which yielded BChl g_F. The construction of a strain producing BChl g_F validates the biosynthetic pathway established for its synthesis and satisfies a precondition for assembling the simplest reaction center in a heterologous organism, namely the biosynthesis of its native pigment, BChl g_F.