Synthesis of chlorophyll a regulates translation of chlorophyll a apoproteins P700, CP47, CP43 and D2 in barley etioplasts

Abstract

Accumulation of plastid-encoded chlorophyll apoproteins and chlorophyll synthesis are controlled by light in angiosperms. An in vitro system utilizing isolated and lysed barley (Hordeum vulgare L.) etioplasts revealed the specific accumulation of P700, CP47, CP43 and D2 triggered by de nova synthesis of chlorophyll. Accumulation rates of radiolabelled chlorophyll apoproteins were linear for about 30 min. Pulse/chase translation assays showed that synthesis of chlorophyll does not result in increased chlorophyll apoprotein stability. Instead turnover rates of chlorophyll apoproteins were higher in the presence than in the absence of chlorophyll. Chlorophyll-dependent accumulation of chlorophyll apoproteins must therefore be regulated on the level of translation. Translation of chlorophyll apoproteins was blocked to about 50% by addition of 30-50 pM aurintricarboxylic acid or 20 pM kasugamycin. The kinetics of chlorophyll-dependent translation indicated that the in vitro translation system is capable of translation initiation. The capability of translation initiation was lost in lysed etioplasts after preincubation for at least 5 min without chlorophyll synthesis. The results suggest that initiation is involved in chlorophyll-dependent regulation of translation.The development of photosynthetically active plastids from proplastids requires activation of a large number of nuclear and plastid genes. A central question, therefore, concerns the nature and origin of signals that initiate and control this process. A key requirement in the control of developmental processes in higher plants is fulfilled by light which acts as environmental control factor regulating the accumulation of some nuclear genes for plastid-localized proteins on such diverse levels as transcription (Fluhr et al., 1986), translation (Slovin and Tobin, 1982) and protein stability (Bennett, 1981).

Chlorophyll Regulates Accumulation of the Plastid-Encoded Chlorophyll Proteins P700 and D1 by Increasing Apoprotein Stability

“…Two inhibitors of bacterial initiation, kasugamycin and lincomycin were tested for their ability to inhibit plastid translation initiation. Kasugamycin inhibits initiation in bacteria by preventing the binding of N-formyl-methionyltRNA to the 30s subunit, and in lysed plastids, kasugamycin inhibits plastid translation initiation (Eichacker et al, 1992). However, kasugamycin did not inhibit translation in intact plastids presumably because of the lack of transport across the plastid envelope.…”

“…In particular, light-induced Chl biosynthesis triggers the accumulation of the chloroplast-encoded Chl proteins (Klein et al, 1988a;Eichacker et al, 1992). Chl does not alter Chl apoprotein mRNA level or the distribution of mRNA in polysomes, indicating that Chl activates Chl protein accumulation posttranscriptionally (Klein et al, 1988b).…”

“…Chl apoprotein accumulation might be activated when Chl binds co-translationally to nascent Chl apoproteins and releases a block in translation elongation (Klein et al, 1988b;Eichacker et al, 1992). Alternatively, Chl may bind to and stabilize the Chl apoproteins, which results in their accumulation.…”

“…Eichacker et al (1992) suggested that light-induced accumulation of the plastidencoded Chl apoproteins may result in part from increased rates of translation initiation. To test this idea, the abundance of translation initiation complexes associated with psbA and psaA in dark-grown and illuminated plants was analyzed.…”

“…Based on these results and pulse labeling-chase experiments, it was suggested that light-mediated Chl synthesis stabilizes nascent Chl apoproteins, which results in their accumulation . It has been reported that Chl-dependent accumulation of Chl proteins may also involve regulation of translation initiation and elongation (Eichacker et al, 1992).…”