Negative regulator of tetrapyrrole biosynthesis (including chlorophyll) in chloroplasts, probably via HEMA1 repression. Inhibits especially the magnesium ion Mg(2+) branch of tetrapyrrole biosynthesis, but independently of heme. Part of the FLU-containing chloroplast membrane complex composed of FLU, CRD1, PORB, PORC, CHLP and HEMA1. Interacts with HEMA1 (via C-terminus) only in the absence of light. No interaction with HEMA2. A number of isoforms are produced. According to EST sequences. Loss of ability to restrict the accumulation of protochloro-phyllide (Pchlide) and delta-amin-olevulinic acid (ALA) in the dark, leading to a strong Pchlide fluorescence in etiolated mutant seedlings exposed to blue light. Rapid bleaching and death of plants transferred from the dark to the light, mediated by singlet oxygen production and enzymatic peroxidation of linolenic acid.