A highly conserved sequence is a novel gene involved inde novovitamin B6 biosynthesis

Abstract

The Cercospora nicotianae SOR1 (singlet oxygen resistance) gene was identified previously as a gene involved in resistance of this fungus to singlet-oxygengenerating phototoxins. Although homologues to SOR1 occur in organisms in four kingdoms and encode one of the most highly conserved proteins yet identified, the precise function of this protein has, until now, remained unknown. We show that SOR1 is essential in pyridoxine (vitamin B6) synthesis in C. nicotianae and Aspergillus flavus, although it shows no homology to previously identified pyridoxine synthesis genes identified in Escherichia coli. Sequence database analysis demonstrated that organisms encode either SOR1 or E. coli pyridoxine biosynthesis genes, but not both, suggesting that there are two divergent pathways for de novo pyridoxine biosynthesis in nature. Pathway divergence appears to have occurred during the evolution of the eubacteria. We also present data showing that pyridoxine quenches singlet oxygen at a rate comparable to that of vitamins C and E, two of the most highly efficient biological antioxidants, suggesting a previously unknown role for pyridoxine in active oxygen resistance.The filamentous, phytopathogenic fungus Cercospora nicotianae exhibits a uniquely effective, broad-spectrum resistance to potent photosensitizers of diverse chemical structure and solubility (1, 2). C. nicotianae is resistant to cercosporin, a light-activated, singlet oxygen ( 1 O 2 )-generating toxin it produces in culture and during plant parasitism, and also to other potent photosensitizers including porphyrins and xanthine and thiazine dyes. Photosensitizers are highly toxic compounds that produce their deleterious effects only after activation by light. Absorbed light energy converts the photosensitizer to an excited (triplet) state molecule that may transfer an electron to oxygen to generate superoxide and͞or transfer energy directly to oxygen, yielding 1 O 2 (3). Exposure of cells to photosensitizers plus light leads to the destruction of critical cellular components including proteins, membranes, and DNA and often results in cell death. Studies on the mechanisms by which organisms protect themselves against reactive oxygen species have focused primarily on reduced and radical forms of oxygen, including hydrogen peroxide (H 2 O 2 ), superoxide (O 2 . ), and the hydroxyl radical (OH⅐). These active oxygen species are byproducts of normal cellular metabolism, and cells contain numerous and conserved defenses against them. By contrast, the highly reactive, but nonradical 1 O 2 is produced primarily via light activation of photosensitizing compounds. Most organisms do not tolerate 1 O 2 , and few biological defenses have been identified (2). The broad-spectrum resistance expressed by Cercospora species against cercosporin and other photosensitizers of diverse structure make these organisms an excellent model for understanding the cellular basis of 1 O 2 resistance. To study specific genes and proteins involved in photosensitizer and 1 O 2 resistance...

Genome-Wide Gene Expression Analysis Reveals a Critical Role for CRYPTOCHROME1 in the Response of Arabidopsis to High Irradiance

“…PDX2 together with PDX1 form a Gln amidotransferase complex involved in vitamin B6 biosynthesis (Tambasco-Studart et al, 2005). Vitamin B6 has been shown to be a potent antioxidant with the ability to quench ROS such as singlet oxygen and superoxide in human erythrocytes (Jain and Lim, 2001) and fungus (Ehrenshaft et al, 1999) and prevents lipid peroxidation in human erythrocytes (Jain and Lim, 2001). Presumably, vitamin B6 could also protect photosynthetic membranes against lipid peroxidation caused by ROS production during high light stress.…”

Genome structure of a Saccharomyces cerevisiae strain widely used in bioethanol production

“…In S288c these regions contain, respectively, the duplicated SNO3 and SNZ3, and SNO2 and SNZ2 genes, which are involved in vitamin B6 metabolism and have a role in oxidative stress tolerance (Padilla et al 1998;Ehrenshaft et al 1999;Rodriguez-Navarro et al 2002). To determine the extent and chromosomal distribution of the SNO/SNZ gene amplifications in JAY270, we did a Southern analysis of pulsed-field gels for this region (Fig.…”

Vitamin B6 biosynthesis in higher plants

“…As pyridoxal 5Ј-phosphate, it is an essential cofactor for numerous metabolic enzymes including amino acid metabolism and antibiotic biosynthesis. Most interestingly, it has recently been found that the vitamin is a potent antioxidant with a particular ability to quench reactive oxygen species such as superoxide and singlet oxygen (1,2). The de novo biosynthesis of vitamin B6 takes place in microorganisms and plants, but this ability has been lost in animals, making it essential in the human diet.…”