Engineered biosynthesis of bacteriochlorophyll g 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, where F is farnesyl, was attempted. Using a strain of Rba. sphaeroides that produces BChl b, where P is phytyl, rather than the native BChl a, 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. Next, the crtE gene was deleted, thereby producing BChl g carrying a THF (tetrahydrofarnesol) moiety. Additionally, the bchG gene from Rba. sphaeroides was replaced with bchG from Hba. modesticaldum. To prevent reduction of the tail, bchP was deleted, which yielded BChl g. The construction of a strain producing BChl g 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.