Protochlorophyllide is found in fruits. Protochlorophyllide is isolated from the seed husks of Cucurbita pepo Chlorophyll itself is bound to proteins and can transfer the absorbed energy in the required direction. Protochlorophyllide, differently, mostly occurs in the free form and under light conditions acts as photosensitizer, forming highly toxic free radicals. Hence plants need an efficient mechanism of regulating the amount of chlorophyll precursor. In angiosperms, this is done at the step of D-Aminolevulinic acid (ALA), one of the intermediate compounds in the biosynthesis pathway. Plants that are fed by ALA accumulate high and toxic levels of protochlorophyllide, so do the mutants with the damaged regulatory system. Despite of numerous past attempts to find the mutant that overacumulates protochlorophyllide under usual conditions, only one such gene (flu) is currently (2009) known. Flu (first described in ) is a nuclear - encoded, chloroplast - located protein that appears containing only protein - protein interaction sites. It is currently not know which other proteins interact through this linker. The regulatory protein is a transmembrane protein that is located in the thylakoid membrane. Later it was discovered that Tigrina mutants in barley, known long time ago, are also mutated in the same gene It is not obvious why no mutants of any other gene were observed; maybe mutations in other proteins, involved into the regulatory chain, are fatal. Flu is a single gene, not a member of the gene family. Protochlorophyllide , more accurate monovinyl protochlorophyllide, is an immediate precursor of chlorophyll a that lacks the phytol side chain of chlorophyll. Unlike chlorophyll, protochlorophyllide is highly fluorescent; mutants that accumulate it glow in red if irradiated by the blue lightIn Angiosperms, the last step, conversion of protochlorophyllide to chlorophyll, is light - dependent and such plants are pale (etiolated) if grown in the darkness. Gymnosperms, algae, and photosynthetic bacteria additionally have another, light - independent enzyme and grow green in the darkness as well. The enzyme that converts protochlorophyllide to chlorophyll is protochlorophyllide reductase , EC 1.3.1.33. There are two structurally unrelated proteins with this activity: the light - dependent and the dark - operative. The light dependent reductase needs light to operate. The dark - operative version is a completely different protein, consisting of three subunits that exhibit significant sequence similarity to the three subunits of nitrogenase, which catalyzes the formation of ammonia from dinitrogen. This enzyme might be evolutionary older but (being similar to nitrogenase) is highly sensitive to free oxygen and does not work if its concentration exceeds about 3 %. Hence the alternative, light dependent version needed to evolve