Recruitment of a Foreign Quinone into the A1 Site of Photosystem I

Abstract

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 the menaquinone biosynthetic pathway in Escherichia coli. Targeted inactivation of the menA and menB genes, which code for phytyl transferase and 1,4-dihydroxy-2-naphthoate synthase, respectively, prevented the synthesis of phylloquinone, thereby confirming the participation of these two gene products in the biosynthetic pathway. The menA and menB mutants grow photoautotrophically under low light conditions (20 E m ؊2 s ؊1 ), with doubling times twice that of the wild type, but they are unable to grow under high light conditions (120 E m ؊2 s ؊1 ). The menA and menB mutants grow photoheterotrophically on media supplemented with glucose under low light conditions, with doubling times similar to that of the wild type, but they are unable 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 mutant 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 fluorescence, P700 photoactivity, and electron transfer rates. PS I complexes isolated from the menA and menB mutant strains contain the full complement 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 flavodoxin. HPLC analyses show high levels of plastoquinone-9 in PS I complexes from the menA and menB mutants but not from the wild type. We propose that in the absence of phylloquinone, PS I recruits plastoquinone-9 into the A 1 site, where it functions as an efficient cofactor in electron transfer from A 0 to the iron-sulfur clusters.

A Plant Locus Essential for Phylloquinone (Vitamin K1) Biosynthesis Originated from a Fusion of Four Eubacterial Genes

“…PCC 6803, providing evidence that PhQ biosynthesis involves almost the same eight enzymatic steps catalyzed by the Men proteins that are required for synthesis of menaquinone, a component of the respiratory chain in eubacteria ( Fig. 1) (3)(4)(5)(6). In Synechocystis PhQ-deficient mutants, the level of active PSI is reduced to about 50 -60% of the wild type (WT), while Photosystem II (PSII) activity remains unaltered.…”

“…In Synechocystis PhQ-deficient mutants, the level of active PSI is reduced to about 50 -60% of the wild type (WT), while Photosystem II (PSII) activity remains unaltered. These mutants grow photoautotrophically under low light conditions because the missing quinone in PSI is functionally replaced by plastoquinone (PQ) (3).…”

Deficiency in Phylloquinone (Vitamin K1) Methylation Affects Prenyl Quinone Distribution, Photosystem I Abundance, and Anthocyanin Accumulation in the Arabidopsis AtmenG Mutant

“…A block in biosynthesis resulting in total loss of phylloquinone has only a minor impact on photosynthesis or growth of Synechocystis under low light conditions, presumably, because other hydroquinones, such as plastoquinone, can largely substitute for phylloquinone deficiency in PSI (7). However, under high light conditions, photoautotrophic growth of Synechocystis menA and menB mutant strains is severely compromised (7).…”

“…The role of phylloquinone in photosynthesis has been studied by employing loss-of-function mutants of cyanobacteria (menA, menB, menD, menE, menG) (7,9,10). A block in biosynthesis resulting in total loss of phylloquinone has only a minor impact on photosynthesis or growth of Synechocystis under low light conditions, presumably, because other hydroquinones, such as plastoquinone, can largely substitute for phylloquinone deficiency in PSI (7).…”

“…The phylloquinone molecule is composed of a naphthoquinone ring, which can exist in the oxidized quinone or in the reduced hydroquinone form, and a prenyl side chain derived from phytyl-diphosphate. The biosynthesis of menaquinone, a bacterial vitamin K analog carrying an unsaturated side chain, and of phylloquinone has been studied in Escherichia coli and in cyanobacteria, respectively (7)(8)(9). The naphthoquinone ring is derived from chorismate, which is converted into 1,4-dihydroxy-2-naphthoate (DHNA) by six consecutive reactions (Fig.…”