Recruitment of a foreign quinone into the A(1) site of photosystem I - I. Genetic and physiological characterization of phylloquinone biosynthetic pathway mutants in Synechocystis sp PCC 6803

Genes encoding enzymes of the biosynthetic pathway leading to phylloquinone , the secondary electron acceptor of photosystem (PS) I, were identified in Synechocystis sp, PCC 6803 by comparison with genes encoding enzymes of th e menaquinone biosynthetic pathway in Escherichia coli, Targeted inactivati on of the menA and menB genes, which code for phytyl transferase and 1,4-di hydroxy-2-naphthoate synthase, respectively, prevented the synthesis of phy lloquinone, thereby confirming the participation of these two gene products in the biosynthetic pathway. The menA and menB mutants grow photoautotroph ically under low light conditions (20 mu E m(-2) s(-1)), with doubling time s twice that of the mild type, but they are unable to grow under high light conditions (120 mu E m(-2) s(-1)). The menA and menB mutants grow photohet erotrophically on media supplemented with glucose under low light condition s, with doubling times similar to that of the wild type, but they are unabl e to grow under high light conditions unless atrazine is present to inhibit PS II activity. The level of active PS II per cell in the menA and menB mu tant strains is identical to that of the wild type, but the level of active PS I is about 50-60% that of the wild type as assayed by low temperature f luorescence, P700 photoactivity, and electron transfer rates. PS I complexe s isolated from the menA and menB mutant strains contain the full complemen t of polypeptides, show photoreduction of F-A and F-B at 15 K, and support 82-84% of the wild type rate of electron transfer from cytochrome c(6) to f lavodoxin, HPLC analyses show high levels of plastoquinone-g in PS I comple xes from the menA and menB mutants but not from the wild type. We propose t hat in the absence of phylloquinone, PS I recruits plastoquinone-g into the A(1) site, where it functions as an efficient cofactor in electron transfe r from A(0) to the iron-sulfur clusters.