The shikimate pathway links metabolism of carbohydrates to biosynthesis of aromatic compounds. In a sequence of seven metabolic steps, phosphoenolpyruvate and erythrose 4-phosphate are converted to chorismate, the precursor of the aromatic amino acids and many aromatic secondary metabolites. All pathway intermediates can also be considered branch point compounds that may serve as substrates for other metabolic pathways. The shikimate pathway is found only in microorganisms and plants, never in animals. All enzymes of this pathway have been obtained in pure form from prokaryotic and eukaryotic sources and their respective DNAs have been characterized from several organisms. The cDNAs of higher plants encode proteins with amino terminal signal sequences for plastid import, suggesting that plastids are the exclusive locale for chorismate biosynthesis. In microorganisms, the shikimate pathway is regulated by feedback inhibition and by repression of the first enzyme. In higher plants, no physiological feedback inhibitor has been identified, suggesting that pathway regulation may occur exclusively at the genetic level. This difference between microorganisms and plants is reflected in the unusually large variation in the primary structures of the respective first enzymes. Several of the pathway enzymes occur in isoenzymic forms whose expression varies with changing environmental conditions and, within the plant, from organ to organ. The penultimate enzyme of the pathway is the sole target for the herbicide glyphosate. Glyphosate-tolerant transgenic plants are at the core of novel weed control systems for several crop plants.

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In Escherichia coli, genes aroF+, aroG+, and aroH+ encode isoenzymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthases that are feedback inhibited by tyrosine, phenylalanine, and tryptophan, respectively. A single base pair change in aroF causes a Pro-148-to-Leu-148 substitution and results in a tyrosine-insensitive enzyme.

As a catalytic cofactor, biotin has a critical role in the enzymological mechanism of a number of enzymes that are essential in both catabolic and anabolic metabolic processes. In this study we demonstrate that biotin has additional non-catalytic functions in regulating gene expression in plants, which are biotin autotrophic organisms. Biotin controls expression of the biotin-containing enzyme, methylcrotonyl-coenzyme A (CoA) carboxylase by modulating the transcriptional, translational and/or posttranslational regulation of the expression of this enzyme. The bio1 mutant of Arabidopsis, which is blocked in the de novo biosynthesis of biotin, was used to experimentally alter the biotin status of this organism. In response to the bio1-associated depletion of biotin, the normally biotinylated A-subunit of methylcrotonyl-CoA carboxylase (MCCase) accumulates in its inactive apo-form, and both MCCase subunits hyperaccumulate. This hyperaccumulation occurs because the translation of each subunit mRNA is enhanced and/or because the each protein subunit becomes more stable. In addition, biotin affects the accumulation of distinct charge isoforms of MCCase. In contrast, in response to metabolic signals arising from the alteration in the carbon status of the organism, biotin modulates the transcription of the MCCase genes. These experiments reveal that in addition to its catalytic role as an enzyme cofactor, biotin has multiple roles in regulating gene expression.Biotin is a water-soluble vitamin biosynthesized by plants, some fungi, and most bacteria and is required by all living organisms for normal cellular functions and growth. Extensive genetic and biochemical studies of prokaryotic organisms have established that biotin is biosynthesized from pimeloyl-CoA and Ala via a four-reaction biosynthetic pathway (DeMoll, 1996). Less extensive studies indicate that plants biosynthesize biotin via an analogous pathway (Shellhammer and Meinke, 1990;Weaver et al., 1995;Patton et al., 1996;Patton et al., 1998; Alban et al., 2000). Of the four enzymes required for biotin biosynthesis, only the one catalyzing the terminal reaction has been molecularly characterized in plants. This enzyme, called biotin synthase, is encoded by the BIO2 gene of Arabidopsis and is a mitochondrial protein (Weaver et al., 1995;Patton et al., 1996; Baldet et al., 1997;Patton et al., 1998). Hence, in plants, biotin is biosynthesized in the mitochondria.Biotin acts as a coenzyme, covalently bound to a Lys residue of a group of enzymes that catalyze carboxylation, decarboxylation or transcarboxylation reactions (Moss and Lane, 1971). The reactions catalyzed by these enzymes are involved in diverse metabolic processes including lipogenesis (acetyl-CoA carboxylase [ACCase]), gluconeogenesis (pyruvate carboxylase), and amino acid metabolism (methylcrotonyl-CoA carboxylase [MCCase] and propionylCoA carboxylase). These enzymes share a common biochemical reaction mechanism, in which the biotin prosthetic group acts as an intermediate carrier of the carboxyl grou...