Chlorophyll biosynthesis in higher plants. Regulatory aspects of 5-aminolevulinate formation

Chlorophyll, heme, phytochrome and siroheme biosynthesis in higher plants is regulated by the supply of their precursor 5-aminolevulinate (ALA), which is derived from glutamate. Glutamate conversion to ALA occurs only in the plastid and requires chloroplast coded tRNA(Glu) and nuclear coded glutamyl-tRNA synthetase, glutamyl-tRNA reductase and glutamate 1-semialdehyde aminotransferase. The chromosomal location in Arabidopsis thaliana, rice and barley of these factors are presented with UniGene data for cDNA tissue locations. Similarly also for other nuclear gene products affecting ALA formation: sigma factor sigB; the A. thaliana Flu and monocot (figrina-d) homologs; xi-carotene desaturase, lycopene synthase and carotenoid isomerase. Cytokinin is a positive regulator of tRNA(Glu) amounts, which correlate with ALA formation. SigB is required for tRNA(Glu) transcription. Phytochrome A, cryptochrome and Mg-proto-porphyrin repress transcription of glutamyl-tRNA reductase in the dark, which is de-repressed by red and blue light. Post-transcriptional control is little understood but may in Poaceae involve messages stabilised by 5'UTR stem-loops. The reductase is inhibited by heme. The Mg-branch has its own inhibitor, A thaliana Flu, encoding a membrane protein, recently found to be identical to barley Tigrina-d. It interacts with glutamyl-tRNA reductase through TPR domains mediating Mg-protoporphyrin inhibition. Carotenoid deficient regulatory tigrina or their phenocopies point to membranes as a regulatory site or to abscisic acid as a negative regulator.