Distinctive features and differential regulation of the <i>DRTS</i> genes of <i>Arabidopsis thaliana</i>

<div><p>In plants and protists, dihydrofolate reductase (DHFR) and thymidylate synthase (TS) are part of a bifunctional enzyme (DRTS) that allows efficient recycling of the dihydrofolate resulting from TS activity. <i>Arabidopsis thaliana</i> possesses three <i>DRTS</i> genes, called <i>AtDRTS1</i>, <i>AtDRTS2</i> and <i>AtDRTS3</i>, that are located downstream of three members of the sec14-like <i>SFH</i> gene family. In this study, a characterization of the <i>AtDRTS</i> genes identified alternatively spliced transcripts coding for AtDRTS isoforms which may account for monofunctional DHFR enzymes supporting pathways unrelated to DNA synthesis. Moreover, we discovered a complex differential regulation of the <i>AtDRTS</i> genes that confirms the expected involvement of the <i>AtDRTS</i> genes in cell proliferation and endoreduplication, but indicates also functions related to other cellular activities. <i>AtDRTS1</i> is widely expressed in both meristematic and differentiated tissues, whereas <i>AtDRTS2</i> expression is almost exclusively limited to the apical meristems and <i>AtDRTS3</i> is preferentially expressed in the shoot apex, in stipules and in root cap cells. The differential regulation of the <i>AtDRTS</i> genes is associated to distinctive promoter architectures and the expression of <i>AtDRTS1</i> in the apical meristems is strictly dependent on the presence of an intragenic region that includes the second intron of the gene. Upon activation of cell proliferation in germinating seeds, the activity of the <i>AtDRTS1</i> and <i>AtDRTS2</i> promoters in meristematic cells appears to be maximal at the G1/S phase of the cell cycle. In addition, the promoters of <i>AtDRTS2</i> and <i>AtDRTS3</i> are negatively regulated through E2F <i>cis</i>-acting elements and both genes, but not <i>AtDRTS1</i>, are downregulated in plants overexpressing the AtE2Fa factor. Our study provides new information concerning the function and the regulation of plant <i>DRTS</i> genes and opens the way to further investigations addressing the importance of folate synthesis with respect to specific cellular activities.</p></div>