The multifaceted regulation of 5-aminolevulinic acid synthesis. Numerous ways to control glutamyl-tRNA reductase

5-Aminolevulinic acid (ALA) is the first stable, committed metabolic intermediate of the tetrapyrrole biosynthesis pathway. The different classes of tetrapyrrole end-products, such as chlorophyll, heme, phycobilins, and siroheme, fulfill pivotal functions in photosynthetic organisms and are required in different quantities during the daily environmental changes and the entire plant development. As result of the light-absorbing characteristics of tetrapyrroles as well as their redox properties as oxidants and reductants, the biosynthetic pathway requires safety precautions to ensure a continuous flow of metabolites and to prevent accumulation of excessive light-absorbing end-products, intermediates, and breakdown products. ALA synthesis is the rate-limiting step of tetrapyrrole biosynthesis and is controlled by tight light-dark and environmental-dependent regulatory mechanisms at the transcriptional and post-translational level. In this chapter, we summarize several post-translational control mechanisms of ALA synthesis, mainly on glutamyl-tRNA reductase, the first enzyme of tetrapyrrole biosynthesis, which ensure a partially coinciding regulation for the adequate supply of metabolic precursors for the synthesis of tetrapyrroles in plants.