PORA and PORB, Two Light-Dependent Protochlorophyllide-Reducing Enzymes of Angiosperm Chlorophyll Biosynthesis.

Abstract

Chloroplasts are the most abundant organelles in a plant cell. Besides their familiar roles in photosynthesis, chloroplasts also perform important functions during numerous other metabolic processes, such as nitrogen assimilation, amino acid biosynthesis, and tetrapyrrole production. ln particular, a major route of chloroplast anabolism is devoted to the synthesis of chlorophyll (Chl; von Wettstein et al., 1995). As the “pigment of life,” Chl plays a fundamental role in the energy absorption and transduction activities of all photosynthetic organisms (Bogorad, 1967; von Wettstein et al., 1971). In angiosperms, Chl synthesis is dependent on light (Granick, 1950). NADPH:protochlorophyllide oxidoreductase (POR; EC 1.3.1.33) catalyzes the only known light-requiring step of tetrapyrrole biosynthesis, the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) (Griffiths, 1978; Apel et al., 1980). Both the POR(A) enzyme and its substrate, Pchlide, accumulate to high levels in dark-grown plants. Together with NADPH, they form a ternary complex that instantaneously converts Pchlide to Chlide when illuminated. The operation of PORA hence helps prevent photodynamic damage by large amounts of not immediately photoconvertible Pchlide. The function of PORA is confined to the very early stages of transition from etiolated to light growth (Apel, 1981; Batschauer and Apel, 1984; Mosinger et al., 1985; Forreiter et al., 1990). The amounts of both PORA protein and porA mRNA decrease drastically soon after the beginning of illumination (Mapleston and Griffiths, 1980; Santel and Apel, 1981; Forreiter et al., 1990), due in part to rapid proteolytic turnover of the enzyme protein (Kay and Griffiths, 1983; Hauser et al., 1984). To perform the reduction of Pchlide to Chlide during the final stages of the light-induced greening and to sustain Chl