29th Annual Meeting of the American Society of Photobiology

Downtown Marriot

Chicago, Il.

Biological Incorporation of Anthraquinones into the A1 Site of Photosystem I

Abstract-
The objective of this work is to use biological quinone incorporation to study the effect of changes in redox potential on the forward electron transfer rates through the quinone, and backward electron transfer rates from [F_A/F_B]^- to P700⁺. We have achieved the necessary precondition to introduce quinones into PS I biologically. The method is to use growth medium supplementation in the menB mutants to produce a phytylated quinone that can displace PQ-9 from the A₁ binding sites. The key to this experiment is that both the menB and menA mutant strains are sensitive to high light levels. However, the menB mutants are insensitive to high light when grown in the presence of a NQ that can be phytylated and incorporated into the A₁ binding sites. We have completed a series of growth experiments with anthraquinones (AQ) and found that in the menB mutant, addition of 9, 10-AQ, 1-NH2-9,10-AQ, 2,6-diNH2-9,10-AQ, 1-CH3NH2-9,10-AQ, 2,3-(CH3)2-9,10-AQ, and 2-tert-butyl-9,10-AQ result in growth to high cell densities in a high-light selection regime. A similar growth screen with identical anthraquinones in the menA mutant resulted in no growth and in cell death. The implication is that a phytylated AQ has displaced PQ-9 from the A₁ binding sites and is functional in electron transfer. In a study of PS I complexes isolated from menB cells grown in the presence of 9,10-AQ, the CW EPR and transient EPR spectrum of the photoaccumulated quinone was narrower than in the wild-type, which is consistent with the presence of an anthraquinone. Not all anthraquinones are effective in conferring high-light resistance to the menB cells. We suspect that these quinones either cannot become phytylated, or if phytylated cannot displace PQ-9 from the A₁ binding sites, or if phytylated and in the A₁ binding sites, the redox potential may be either too low or too high to support forward electron transfer. Our experimental approach is to first verify the presence of a substituted AQ in PS I by mass spectroscopy and EPR spectroscopy followed by measurement the rates of forward and backward electron transfer using transient EPR and time-resolved optical spectroscopy. Supported by the NSF.

Keywords: Photosystem I, Phylloquinone, mutant, quinone