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Abstract

The analysis of clones obtained by rapid amplification of the 5' end and by primer extension of the mRNA for carrot bifunctional dihydrofolate reductase-thymidylate synthase showed transcripts of differing lengths that belonged to two sub-populations. The longer transcripts were found to contain a translation start site 147 nt upstream of, and in frame with, the one which is present in the shorter transcripts. The ORF that begins at this ATG codes for a protein of 64714 Da, which is much larger than mature DHFR-TS subunit. The N-terminus region of this polypeptide shows features typical of plant transit peptides. Immunogold labelling studies and immunorecognition of the plastid-containing sub-cellular fraction suggested a plastidial localisation of the bifunctional protein. Although plant cells were shown to contain folate pools in plastids, in mitochondria and in the cytosol, few enzymes of the folate pathway have been associated with any sub-cellular compartment. Thus, this is the first indication for the presence of an enzyme of the folate biosynthetic pathway in plastids. The longer transcripts revealed the presence of a TC microsatellite at the 5'-untranslated end.

Tetrahydrofolate biosynthesis in plants: Molecular and functional characterization of dihydrofolate synthetase and three isoforms of folylpolyglutamate synthetase in Arabidopsis thaliana

“…7). Also, the presence of DHFR-TS in the plastids (31) and FPGS in the cytosol (16) have been reported.…”

“…In plants, the traffic of oxidized folates would imply the presence of dihydrofolate reductase-thymidylate synthase (DHFR-TS) isoenzymes in the cytosol and the chloroplasts to reduce dihydro-to tetrahydrofolate before the addition of glutamate residues by FPGS. The presence of a DHFR-TS in the plastids has been suggested by immunological techniques (31), whereas the existence of a cytosolic isoform is supported only by DNA sequence data. The intracellular transport of reduced folates is also questionable because of their sensitivity to oxidative degradation (40).…”

Folates in Plants: Research Advances and Progress in Crop Biofortification

“…In bacteria, yeast and vertebrates, DHFR is a monofunctional enzyme. The enzyme was demonstrated to be localized in mitochondria and plastids in carrot and pea (Neuburger et al, 1996; Luo et al, 1997). …”

UPS1 and UPS2 from Arabidopsis Mediate High Affinity Transport of Uracil and 5-Fluorouracil

“…Thus, it is likely that both proteins are necessary for effective 5-FU transport from the external medium to the location where the toxic effect occurs, presumably in a sequential manner. According to the functions hypothesized above, AtUPS2 may allow 5-FU allocation in the plant, whereas AtUPS1 may transport it in cotyledon cells with photosynthetic capacity, where the toxic effects of 5-FU are maximal due to inhibition of the plastidic enzyme thymidylate synthetase (26).…”