FLU (plant gene)

Protochlorophyllide, or monovinyl protochlorophyllide, is an immediate precursor of chlorophyll a that lacks the phytol side-chain of chlorophyll. Protochlorophyllide is highly fluorescent; mutants that accumulate it glow red if irradiated with blue light. In angiosperms, the last step, conversion of protochlorophyllide to chlorophyll, is light-dependent, and such plants are pale (chlorotic) if grown in the darkness. Gymnosperms, algae, and photosynthetic bacteria have another, light-independent enzyme and grow green in the darkness as well.

The enzyme that converts protochlorophyllide to chlorophyllide is ,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.

Most of the photosynthetic bacteria have both light-dependent and light-independent reductases. Angiosperms have lost the dark-operative form and rely on 3 slightly different copies of light-dependent version, frequently abbreviated as POR A, B, and C. Gymnosperms have much more copies of the similar gene (Loblolly pine has about 11 [9]). In plants, POR is encoded in the cell nucleus and only later transported to its place of work, chloroplast. Unlike with POR, in plants and algae that have the dark-operative enzyme it is at least partially encoded in the chloroplast genome.

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