Jy. Suzuki et Ce. Bauer, A PROKARYOTIC ORIGIN FOR LIGHT-DEPENDENT CHLOROPHYLL BIOSYNTHESIS OF PLANTS, Proceedings of the National Academy of Sciences of the United Statesof America, 92(9), 1995, pp. 3749-3753
Flowering plants require light for chlorophyll synthesis, Early studie
s indicated that the dependence on light for greening stemmed in part
from the light-dependent reduction of the chlorophyll intermediate pro
tochlorophyllide to the product chlorophyllide. Light-dependent reduct
ion of protochlorophyllide by flowering plants is contrasted by the ab
ility of nonflowering plants, algae, and photosynthetic bacteria to re
duce protochlorophyllide and, hence, synthesize (bacterio)chlorophyll
in the dark, In this report, we functionally complemented a light-inde
pendent protochlorophyllide reductase mutant of the eubacterium Rhodob
acter capsulatus with an expression library composed of genomic DNA fr
om the cyanobacterium Synechocystis sp, PCC 6803. The complemented R.
capsulatus strain is capable of synthesizing bacteriochlorophyll in th
e light, thereby indicating that a chlorophyll biosynthesis enzyme can
function in the bacteriochlorophyll biosynthetic pathway, However, un
der dark growth conditions the complemented R, capsulatus strain fails
to synthesize bacteriochlorophyll and instead accumulates protochloro
phyllide. Sequence analysis demonstrates that the complementing Synech
ocystis genomic DNA fragment exhibits a high degree of sequence identi
ty (53-56%) with light-dependent protochlorophyllide reductase enzymes
found in plants, The observation that a plant-type, light-dependent p
rotochlorophyllide reductase enzyme exists in a cyanobacterium indicat
es that light-dependent protochlorophyllide reductase evolved before t
he advent of eukaryotic photosynthesis, As such, this enzyme did not a
rise to fulfill a function necessitated either by the endosymbiotic ev
olution of the chloroplast or by multicellularity; rather, it evolved
to fulfill a fundamentally cell-autonomous role.