A NEW-TYPE OF COMPLEMENTARY CHROMATIC ADAPTATION EXEMPLIFIED BY PHORMIDIUM SP C86 - CHANGES IN THE NUMBER OF PERIPHERAL RODS AND IN THE STOICHIOMETRY OF CORE COMPLEXES IN PHYCOBILISOMES

The marine cyanobacterium Phormidium sp. strain C86 changes the phycob ilisome type depending on light quality. Red-light-adapted cells conta ined hemidiscoidal phycobilisomes with a photosystem II:phycobilisome ratio of 2.2, while green-light-adapted cells exhibited hemiellipsoida l phycobilisomes with a photosystem II:phycobilisome ratio of 4.4, as determined by a combined analysis of freeze-fractured thylakoid membra nes and ultrathin sections and by photochemical determinations of phot osystems and phycobilisomes. Core complexes of phycobilisomes of red- and green-light-adapted cells were isolated by affinity chromatography and were subsequently separated into two allophycocyanin-containing f ractions. The high-molecular-weight fraction, with a sedimentation coe fficient of 23 S and a calculated mol. wt. of 860,000, contained compl exes of the quaternary structure (alpha(9)(AP) beta(8)(AP) beta(19.5AP ))(2) .(L(CM))(2) and tricylindrical shape, previously designated AP(C M). This fraction was similar in size in red- and green-light-adapted cells; however, differences were detected in the low-molecular-weight allophycocyanin fraction containing the ''trimeric'' complexes with a sedimentation coefficient of 6 S. As shown by comparison of spectral a nd stoichiometric data of intact phycobilisomes and isolated core comp lexes, the amount of the alpha(APB)-containing core (alpha(2)(AP)alpha (APB)beta(3)(AP)). L(C)(10) greater in core fractions of green-light p hycobilisomes, whereas the amount of the core complexes (alpha(3)(AP)b eta(3)(AP)).(10)(C), designated AP . L(C)(10), was higher in cores of red-light phycobilisomes. Phormidium sp. is the first organism examine d that exhibits a new type of complementary chromatic adaptation by al tering the composition of the phycobilisome core and the number and co mposition of peripheral rods and by changing the ratio of photosystem II to phycobilisomes. A model summarizing the structural consequences of the results is presented.