H. Holtorf et al., 2 ROUTES OF CHLOROPHYLLIDE SYNTHESIS THAT ARE DIFFERENTIALLY REGULATED BY LIGHT IN BARLEY (HORDEUM-VULGARE L), Proceedings of the National Academy of Sciences of the United Statesof America, 92(8), 1995, pp. 3254-3258
NADPH-protochlorophyllide oxidoreductase (POR; EC 1.6.99.1) catalyzes
the only known light-dependent step in chlorophyll synthesis of higher
plants, the reduction of protochlorophyllide (Pchlide) to chlorophyll
ide. In barley, two distinct immunoreactive POR proteins were identifi
ed, In contrast to the light-sensitive POR enzyme studied thus far (PO
R-A), levels of the second POR protein remained constant in seedlings
during the transition from dark growth to the light and in green plant
s. The existence of a second POR-related protein was verified by isola
ting and sequencing cDNAs that encode a second POR polypeptide (POR-B)
with an amino acid sequence identity of 75% to the POR-A. In the pres
ence of NADPH and Pchlide, the in vitro-synthesized POR-A and POR-B pr
oteins could be reconstituted to ternary enzymatically active complexe
s that reduced Pchlide to chlorophyllide only after illumination. Even
though the in vitro activities of the two enzymes were similar, the e
xpression of their genes during the light-induced transformation of et
iolated to green seedlings was distinct. While the POR-A mRNA rapidly
declined during illumination of dark-grown seedlings and soon disappea
red, POR-B mRNA remained at an approximately constant level in dark-gr
own and green seedlings. Thus these results suggest that chlorophyll s
ynthesis is controlled by two light-dependent POR enzymes, one that is
active only transiently in etiolated seedlings at the beginning of il
lumination and the other that also operates in green plants.