title: A prokaryotic origin for light-dependent chlorophyll biosynthesis of plants.
abstract: Flowering plants require light for chlorophyll synthesis. Early studies indicated that the dependence on light for greening stemmed in part from the light-dependent reduction of the chlorophyll intermediate protochlorophyllide to the product chlorophyllide. Light-dependent reduction of protochlorophyllide by flowering plants is contrasted by the ability of nonflowering plants, algae, and photosynthetic bacteria to reduce protochlorophyllide and, hence, synthesize (bacterio) chlorophyll in the dark. In this report, we functionally complemented a light-independent protochlorophyllide reductase mutant of the eubacterium Rhodobacter capsulatus with an expression library composed of genomic DNA from the cyanobacterium Synechocystis sp. PCC 6803. The complemented R. capsulatus strain is capable of synthesizing bacteriochlorophyll in the light, thereby indicating that a chlorophyll biosynthesis enzyme can function in the bacteriochlorophyll biosynthetic pathway. However, under dark growth conditions the complemented R. capsulatus strain fails to synthesize bacteriochlorophyll and instead accumulates protochlorophyllide. Sequence analysis demonstrates that the complementing Synechocystis genomic DNA fragment exhibits a high degree of sequence identity (53-56%) with light-dependent protochlorophyllide reductase enzymes found in plants. The observation that a plant-type, light-dependent protochlorophyllide reductase enzyme exists in a cyanobacterium indicates that light-dependent protochlorophyllide reductase evolved before the advent of eukaryotic photosynthesis. As such, this enzyme did not arise to fulfill a function necessitated either by the endosymbiotic evolution of the chloroplast or by multicellularity; rather, it evolved to fulfill a fundamentally cell-autonomous role.
authors.affiliation: Department of Biology, Indiana University, Bloomington 47405, USA.
authors.forename: J Y
authors.initials: JY
authors.lastname: Suzuki
authors.forename: C E
authors.initials: CE
authors.lastname: Bauer
mesh_terms: D000595:Amino Acid Sequence; D002734:Chlorophyll / Q000096:biosynthesis*; D003001:Cloning, Molecular; D003360:Cosmids; D000458:Cyanobacteria / Q000201:enzymology / Q000235:genetics; D015723:Gene Library; D005816:Genetic Complementation Test; D008027:Light; D008969:Molecular Sequence Data; D016366:Open Reading Frames; D010088:Oxidoreductases / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism; D044925:Oxidoreductases Acting on CH-CH Group Donors; D010944:Plants / Q000201:enzymology* / Q000235:genetics; D010957:Plasmids; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry; D015183:Restriction Mapping; D016354:Rhodobacter capsulatus / Q000201:enzymology / Q000235:genetics; D017386:Sequence Homology, Amino Acid
chemical_list: D011994:Recombinant Proteins; D002734:Chlorophyll; D010088:Oxidoreductases; D044925:Oxidoreductases Acting on CH-CH Group Donors; C023886:protochlorophyllide reductase