Enzymes of tyrosine catabolism in Arabidopsis thaliana.

Tyrosine catabolism is an essential pathway in animals, but its role in plants is unclear. The first steps of tyrosine degradation lead to the formation of homogentisate. In animals this is then sequentially acted on by homogentisate dioxygenase (HGO), maleylacetoacetate isomerase (MAAI) and fumarylacetoacetate hydrolase (FAH) to generate fumarate and acetoacetate. In plants, homogentisate is used to generate the essential redox metabolites tocopherol and plastoquinone, which effectively act as an alternative metabolic fate for tyrosine. Having determined that a zeta class glutathione transferase from Arabidopsis thaliana is a functional MAAI, we have looked for evidence that the mammalian degradation pathway could also operate in plants. Based on array and quantitative PCR experiments, the A. thaliana homologues AtHGO, AtMAAI and AtFAH could be shown to be expressed, with AtHGO and AtMAAI showing evidence of co-regulation. cDNAs encoding AtHGO, AtMAAI and AtFAH were cloned in Escherichia coli and shown to represent a fully functional catabolic pathway when combined in vitro. The significance of this pathway, including increased transcription of the associated enzymes in senescing tissue, compartmentalisation and impact on flux into synthesis of Vitamin E and other tocopherols of biotechnological interest is discussed.