Posttranslational influence of NADPH-dependent thioredoxin reductase C on enzymes in tetrapyrrole synthesis.

The NADPH-dependent thioredoxin reductase C (NTRC) is involved in redox-related regulatory processes in chloroplasts and nonphotosynthetic active plastids. Together with 2-cysteine peroxiredoxin, it forms a two-component peroxide-detoxifying system that acts as a reductant under stress conditions. NTRC stimulates in vitro activity of magnesium protoporphyrin IX monomethylester (MgPMME) cyclase, most likely by scavenging peroxides.

Methods for analysis of photosynthetic pigments and steady-state levels of intermediates of tetrapyrrole biosynthesis.

Tetrapyrroles and carotenoids are required for many indispensable functions in photosynthesis. Tetrapyrroles are essential metabolites for photosynthesis, redox reaction, and detoxification of reactive oxygen species and xenobiotics, while carotenoids function as accessory pigments, in photoprotection and in attraction to animals. Their branched metabolic pathways of synthesis and degradation are tightly controlled to provide adequate amounts of each metabolite (carotenoids/tetrapyrroles) and to prevent accumulation of photoreactive intermediates (tetrapyrroles).

Mg protoporphyrin monomethylester cyclase deficiency and effects on tetrapyrrole metabolism in different light conditions.

Mg protoporphyrin monomethylester (MgProtoME) cyclase catalyzes isocyclic ring formation to form divinyl protochlorophyllide. The CHL27 protein is part of the cyclase complex. Deficiency of CHL27 has been previously reported to compromise photosynthesis and nuclear gene expression.

Rapid dark repression of 5-aminolevulinic acid synthesis in green barley leaves.

In photosynthetic organisms chlorophyll and heme biosynthesis is tightly regulated at various levels in response to environmental adaptation and plant development. The formation of 5-aminolevulinic acid (ALA) is the key regulatory step and provides adequate amounts of the common precursor molecule for the Mg and Fe branches of tetrapyrrole biosynthesis. Pathway control prevents accumulation of metabolic intermediates and avoids photo-oxidative damage.

GUN4 is required for posttranslational control of plant tetrapyrrole biosynthesis.

In aerobic photosynthetic organisms, GUN4 binds the chlorophyll intermediates protoporphyrin and Mg protoporphyrin, stimulates Mg chelatase activity, and is implicated in plastidic retrograde signaling. GUN4 expression is most abundant in young and greening tissues and parallels the activity of 5-aminolevulinic acid (ALA) ALA and Mg porphyrin biosynthesis during photoperiodic growth. We explored function and mode of action of GUN4 using GUN4-deficient and overexpressing plants.

Expression of chlorophyll synthase is also involved in feedback-control of chlorophyll biosynthesis.

At the last step of the chlorophyll biosynthetic pathway chlorophyll synthase (CHLG) esterifies chlorophyllide a and b with phytyl or geranyl-geranyl pyrophosphate in chloroplasts. Transgenic tobacco plants expressing CHLG RNA in sense and antisense orientation were examined for the effects of excessive and reduced ectopic CHLG expression, respectively, on the chlorophyll biosynthetic pathway and the expression of chlorophyll-binding proteins. Reduced chlorophyll synthase activity does not result in accumulation of chlorophyllide and caused reduced ALA formation and Mg and ferrochelatase activity, while CHLG overexpression correlated with enhanced ALA synthesizing capacity and more chelatase activities.

Differential requirement of two homologous proteins encoded by sll1214 and sll1874 for the reaction of Mg protoporphyrin monomethylester oxidative cyclase under aerobic and micro-oxic growth conditions.

The two open reading frames in the Synechocystis sp. PCC 6803 genome, sll1214 and sll1874, here designated cycI and cycII, respectively, encode similar proteins, which are involved in the Mg protoporphyrin monomethylester (MgProtoME) cyclase reaction. The impairment of tetrapyrrole biosynthesis was examined by separate inactivation of both cyclase encoding genes followed by analysis of chlorophyll contents, MgProtoME levels and several enzyme activities of tetrapyrrole biosynthesis.

Importance of the cyanobacterial Gun4 protein for chlorophyll metabolism and assembly of photosynthetic complexes.

Gun4 is a porphyrin-binding protein that activates magnesium chelatase, a multimeric enzyme catalyzing the first committed step in chlorophyll biosynthesis. In plants, GUN4 has been implicated in plastid-to-nucleus retrograde signaling processes that coordinate both photosystem II and photosystem I nuclear gene expression with chloroplast function. In this work we present the functional analysis of Gun4 from the cyanobacterium Synechocystis sp.