Recruitment of a Foreign Quinone into the A1 Site of Photosystem I IN VIVO REPLACEMENT OF PLASTOQUINONE-9 BY MEDIA-SUPPLEMENTED NAPHTHOQUINONES IN PHYLLOQUINONE BIOSYNTHETIC PATHWAY MUTANTS OF SYNECHOCYSTIS sp. PCC 6803

Abstract Interruption of the phylloquinone (PhQ) biosynthetic pathway by interposon mutagenesis of themenA and menB genes inSynechocystis sp. PCC 6803 results in plastoquinone-9 (PQ-9) occupying the A1 site and functioning in electron transfer from A0 to the FeS clusters in photosystem (PS) I (Johnson, T. W., Shen, G., Zybailov, B., Kolling, D., Reategui, R., Beauparlant, S., Vassiliev, I. R., Bryant, D. A., Jones, A. D., Golbeck, J. H., and Chitnis, P. R. (2000)J. Biol. Chem. 275, 8523–8530. We report here the isolation of menB26, a strain of the menBmutant that grows in high light by virtue of a higher PS I to PS II ratio. PhQ can be reincorporated into the A1 site of themenB26 mutant strain by supplementing the growth medium with authentic PhQ. The reincorporation of PhQ also occurs in cells that have been treated with protein synthesis inhibitors, consistent with a displacement of PQ-9 from the A1 site by mass action. The doubling time of the menB26 mutant cells, but not the menA mutant cells, approaches the wild type when the growth medium is supplemented with naphthoquinone (NQ) derivatives such as 2-CO2H-1,4-NQ and 2-CH3-1,4-NQ. Since PhQ replaces PQ-9 in the supplemented menB26 mutant cells, but not in the menA mutant cells, the phytyl tail accompanies the incorporation of these quinones into the A1site. Studies with menB26 mutant cells and perdeuterated 2-CH3-1,4-NQ shows that phytylation occurs at position 3 of the NQ ring because the deuterated 2-methyl group remains intact. Therefore, the specificity of the phytyltransferase enzyme is selective with respect to the group present at ring positions 2 and 3. Supplementing the growth medium of menB26 mutant cells with 1,4-NQ also leads to its incorporation into the A1 site, but typically without either the phytyl tail or the methyl group. These findings open the possibility of biologically incorporating novel quinones into the A1 site by supplementing the growth medium of menB26 mutant cells.