Plants need tight regulation of photosynthetic electron transport for survival and growth under environ- mental and metabolic conditions. For this purpose, the linear electron transport （LET） pathway is supple- ment...Plants need tight regulation of photosynthetic electron transport for survival and growth under environ- mental and metabolic conditions. For this purpose, the linear electron transport （LET） pathway is supple- mented by a number of alternative electron transfer pathways and valves. In Arabidopsis, cyclic electron transport （CET） around photosystem I （PSI）, which recycles electrons from ferrodoxin to plastoquinone, is the most investigated alternative route. However, the interdependence of LET and CET and the relative importance of CET remain unclear, largely due to the difficulties in precise assessment of the contribution of CET in the presence of LET, which dominates electron flow under physiological conditions. We there- fore generated Arabidopsis mutants with a minimal water-splitting activity, and thus a low rate of LET, by combining knockout mutations in Psb01, PsbP2, PsbQ1, PsbQ2, and PsbR loci. The resulting 45 mutant is viable, although mature leaves contain only ~20% of wild-type naturally less abundant Psb02 protein. 45 plants compensate for the reduction in LET by increasing the rate of CET, and inducing a strong non-photochemical quenching （NPQ） response during dark-to-light transitions. To identify the molecular origin of such a high-capacity CET, we constructed three sextuple mutants lacking the qE component of NPQ （45 npq4-1）, NDH-mediated CET （45 crr4-3）, or PGR5-PGRLl-mediated CET （45 pgrS）. Their analysis revealed that PGR5-PGRLl-mediated CET plays a major role in ~pH formation and induction of NPQ in C3 plants. Moreover, while pgr5 dies at the seedling stage under fluctuating light conditions, 45 pgr5 plants are able to survive, which underlines the importance of PGR5 in modulating the intersystem electron transfer.展开更多
Rubisco and fructose-1,6-bisphosphate aldolases （FBAs） are involved in CO2 fixation in chloroplasts. Both enzymes are trimethylated at a specific lysine residue by the chloroplastic protein methyltransferase LSMT. G...Rubisco and fructose-1,6-bisphosphate aldolases （FBAs） are involved in CO2 fixation in chloroplasts. Both enzymes are trimethylated at a specific lysine residue by the chloroplastic protein methyltransferase LSMT. Genes coding LSMT are present in all plant genomes but the methylation status of the substrates varies in a species-specific manner. For example, chloroplastic FBAs are naturally trimethylated in both Pisum sati- vum and Arabidopsis thaliana, whereas the Rubisco large subunit is trimethylated only in the former spe- cies. The in vivo methylation status of aldolases and Rubisco matches the catalytic properties of AtLSMT and PsLSMT, which are able to trimethylate FBAs or FBAs and Rubisco, respectively. Here, we created chimera and site-directed mutants of monofunctional AtLSMT and bifunctional PsLSMT to identify the molecular determinants responsible for substrate specificity. Our results indicate that the His-Ala/Pro- Trp triad located in the central part of LSMT enzymes is the key motif to confer the capacity to trimethylate Rubisco. Two of the critical residues are located on a surface loop outside the methyltransferase catalytic site. We observed a strict correlation between the presence of the triad motif and the in vivo methylation status of Rubisco. The distribution of the motif into a phylogenetic tree further suggests that the ancestral function of LSMT was FBA trimethylation. In a recent event during higher plant evolution, this function evolved in ancestors of Fabaceae, Cucurbitaceae, and Rosaceae to include Rubisco as an additional substrate to the archetypal enzyme. Our study provides insight into mechanisms by which SET-domain protein methyltransferases evolve new substrate specificity.展开更多
文摘Plants need tight regulation of photosynthetic electron transport for survival and growth under environ- mental and metabolic conditions. For this purpose, the linear electron transport （LET） pathway is supple- mented by a number of alternative electron transfer pathways and valves. In Arabidopsis, cyclic electron transport （CET） around photosystem I （PSI）, which recycles electrons from ferrodoxin to plastoquinone, is the most investigated alternative route. However, the interdependence of LET and CET and the relative importance of CET remain unclear, largely due to the difficulties in precise assessment of the contribution of CET in the presence of LET, which dominates electron flow under physiological conditions. We there- fore generated Arabidopsis mutants with a minimal water-splitting activity, and thus a low rate of LET, by combining knockout mutations in Psb01, PsbP2, PsbQ1, PsbQ2, and PsbR loci. The resulting 45 mutant is viable, although mature leaves contain only ~20% of wild-type naturally less abundant Psb02 protein. 45 plants compensate for the reduction in LET by increasing the rate of CET, and inducing a strong non-photochemical quenching （NPQ） response during dark-to-light transitions. To identify the molecular origin of such a high-capacity CET, we constructed three sextuple mutants lacking the qE component of NPQ （45 npq4-1）, NDH-mediated CET （45 crr4-3）, or PGR5-PGRLl-mediated CET （45 pgrS）. Their analysis revealed that PGR5-PGRLl-mediated CET plays a major role in ~pH formation and induction of NPQ in C3 plants. Moreover, while pgr5 dies at the seedling stage under fluctuating light conditions, 45 pgr5 plants are able to survive, which underlines the importance of PGR5 in modulating the intersystem electron transfer.
文摘Rubisco and fructose-1,6-bisphosphate aldolases （FBAs） are involved in CO2 fixation in chloroplasts. Both enzymes are trimethylated at a specific lysine residue by the chloroplastic protein methyltransferase LSMT. Genes coding LSMT are present in all plant genomes but the methylation status of the substrates varies in a species-specific manner. For example, chloroplastic FBAs are naturally trimethylated in both Pisum sati- vum and Arabidopsis thaliana, whereas the Rubisco large subunit is trimethylated only in the former spe- cies. The in vivo methylation status of aldolases and Rubisco matches the catalytic properties of AtLSMT and PsLSMT, which are able to trimethylate FBAs or FBAs and Rubisco, respectively. Here, we created chimera and site-directed mutants of monofunctional AtLSMT and bifunctional PsLSMT to identify the molecular determinants responsible for substrate specificity. Our results indicate that the His-Ala/Pro- Trp triad located in the central part of LSMT enzymes is the key motif to confer the capacity to trimethylate Rubisco. Two of the critical residues are located on a surface loop outside the methyltransferase catalytic site. We observed a strict correlation between the presence of the triad motif and the in vivo methylation status of Rubisco. The distribution of the motif into a phylogenetic tree further suggests that the ancestral function of LSMT was FBA trimethylation. In a recent event during higher plant evolution, this function evolved in ancestors of Fabaceae, Cucurbitaceae, and Rosaceae to include Rubisco as an additional substrate to the archetypal enzyme. Our study provides insight into mechanisms by which SET-domain protein methyltransferases evolve new substrate specificity.
