2 ROUTES OF CHLOROPHYLLIDE SYNTHESIS THAT ARE DIFFERENTIALLY REGULATED BY LIGHT IN BARLEY (HORDEUM-VULGARE L)

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.