Photochromic Pigments from Blue-Green Algae: Phycochromes a, b, and c

Aqueous extracts of blue-green algae were fractionated by electrofocusing. In all algae investigated, fractions with iso-electric points at or near 4.6 showed photochromic behaviour analogous to that of phytochrome, although they were sensitive to light of shorter wavelength. Three main types of photochromic pigments were found: Phycochrome a (in Tolypothrix distorta, Phormidium luridum, Nostoc muscorum 1453/12, and Anacystis nidulans) has one form absorbing maximally at about 590 nm (formed under red light) and one absorbing maximally at about 630 nm (formed under green light). Phycochrome b (in Tolypothrix distorta) has one form absorbing maximally near 510 nm and one form absorbing maximally at 570 nm (formed in yellow-green and blue-green light, respectively). Phycochrome c (in Nostoc muscorum A and probably in Tolypothrix tenuis) has one form absorbing maximally at 650 nm (formed under green light) and one absorbing very weakly in the green region (formed under red light). The conversion of Phormidium phycochrome a from its red-absorbing form to its green-absorbing form causes the same spectral change as if an f-chromophore of phycocyanin were transformed into an s-chromophore. The quantum yield for this conversion is estimated to be 0.1, while the quantum yield for the reversion is estimated to be 0.4 on the assumption that the absorption coefficients are those of f- and s-chromophores. Phycochrome c is less light-sensitive than phycochromes a and b.