Multiple forms of P700-chlorophyll a-protein complexes from Synechococcus sp.: The iron, quinone and carotenoid contents.

The iron, quinone and carotenoid contents of five P700-chlorophyll a-protein complexes having different subunit structures (CP1-a,-b,-c,-d and-e) from the thermophilic cyanobacterium Synechococcus sp. were determined. CP1-a,-b,-c and-d that commonly have four polypeptides of 62,000, 60,000, 14,000 and 10,000 dalton contained 10-14 iron atoms per P700, whereas CP1-e that lacks the two small polypeptides was totally devoid of iron. All CP1 complexes contained vitamin K1 at the molar ratio of vitamin K1 to P700 of about 2 except CP1-e that had only 0.4 vitamin K1 per P700. No plastoquinone was detected in five CP1 complexes. Out of four major carotenoids, β-carotene, zeaxanthin, caloxanthin, and myxoxanthophyll, present in the thylakoid membranes, only β-carotene was found in isolated CP1 complexes; all CP1 complexes contained about 10 β-carotene molecules per P700. The flourescence excitation spectrum showed that β-carotene serves as an efficient antenna of photosystem I. It is concluded that all iron atoms and a larger fraction of vitamin K1 molecules present in the photosystem I reaction center complex are associated with the 14,000 and 10,000 dalton polypeptides, whereas β-carotene exclusively binds to the large polypeptides which carry the functional and antenna chlorophyll a. The possible functions of iron and vitamin K1 as electron carriers and of β-carotene as the accessary pigment and a photoprotectant in the photosystem I complexes are discussed.