Protochlorophyllide reduction: a key step in the greening of plants

The reduction of Protochlorophyllide (Pchlide) is a major regulatory step in the biosynthesis of chlorophyll (Chl) in oxygenic phototrophs. Two different enzymes catalyze this reduction: a light-dependent enzyme (LPOR), which is unique as a consequences of its direct utilization of light for catalysis; and a light-independent Pchlide reductase (DPOR). Since the reduction of Pchlide in angiosperms is catalyzed exclusively by LPOR, they become etiolated in the absence of light. LPOR, a major protein in etioplast membranes, consists of a single polypeptide and it exists as a ternary complex with its substrates, Pchlide and NADPH. By contrast to the copious information about LPOR, limited information about DPOR has been reported. Recent molecular genetic analyses in a cyano-bacterium and a green alga have revealed that at least the three genes, namely, chlL, chlN and chlB, encode proteins essential for the activity of DPOR. These genes are widely distributed among phototrophic organisms with exception of angiosperms and Euglenophyta. This distribution seems to be well correlated with light-independent greening ability. These genes might have been lost during the evolution of gymnosperms to angiosperms. The similarities among the deduced amino acid sequences of the three gene products and the subunits of nitrogenase suggest an evolutionary relationship between DPOR and nitrogenase. The identification of genes for the reduction of Pchlide provides the groundwork for investigations of the mechanism that regulates the synthesis of Chl, which is closely coordinated with greening in plants.