Thesis presented April 28, 2004
Abstract:
Tetrahydrofolate (H4F) is a central cofactor for one-carbon transfer reactions in all living organisms.
De novo synthesis of this vitamin occurs only in plants, fungi, and certain bacteria.
In this study, we analyzed the expression of the enzymes hydroxymethyldihydropterin pyrophosphokinase/dihydropteroate synthase (HPPK/DHPS) and dihydrofolate reductase/ thymidylate synthase (DHFR/TS) in pea (Pisum sativum L.) organs during development, and so the enzymatic capacity for
de novo synthesis of H4F. During seedling development, all the organs/tissues examined contain H4F coenzymes and express the two enzymes, thus suggesting that each organ/tissue is autonomous for the synthesis of H4F. During organ differentiation, folate is synthesized preferentially in highly dividing tissues and in photosynthetic leaves. In germinating embryos and in meristematic tissues, the high capacity to synthesize and accumulate folate correlates with the general resumption of cell metabolism and the high requirement for nucleotide synthesis, major cellular processes involving H4F coenzymes. The particular status of folate synthesis in leaves is related to photosynthesis and photorespiration, and thus is dependent on light. In green leaves, the accumulation of folate occurs mainly in the cytosol and seems related to its role in the synthesis of methionine (Met), the precursor of the universal methyl group donor S-adenosylmethionine.
The study carried out with
Arabidopsis thaliana cell suspension cultures confirms the close link between the syntheses of H4F and Met. Indeed, a strong activity of Met synthesis is correlated with a high concentration of folate, and
vice versa. An increase in the intracellular folate content, obtained by the addition of 5-CHO-H4F to the culture medium, seems to influence only the expression of the last step in the synthesis of H4F, which is located in mitochondria. On the opposite, the marked decrease in folate obtained by the addition of specific inhibitors leads to an increase in the amount of the enzymes involved in the synthesis of H4F, and to an original post-translational modification of cystathionine γ-synthase, the first enzyme of
de novo Met synthesis. The rationale of these modifications is probably to increase simultaneously the syntheses of H4F and Met.
Keywords:
Tetrahydrofolate biosynthesis, One-carbon (C1) metabolism, Methionine biosynthesis, Germination and plant development