Studies on the intron-dependent meristematic expression of the dihydrofolate reductase/thymidylate synthase (DRTS) genes of Arabidopsis thaliana

In plants, dihydrofolate reductase (DHFR) and thymidylate synthase (TS) constitute a bifunctional enzyme (DRTS) which is essential for the synthesis of DNA precursors in proliferating or endoreduplicating cells. Previous characterizations of the three AtDRTS genes of Arabidopsis thaliana revealed distinctive patterns of expression and a crucial positive control of AtDRTS1 expression in apical meristems by an intragenic region containing the second intron of the gene [1]. In this study we report that the strong meristematic expression of AtDRTS2 is also strictly dependent on the first intron located in the 5’UTR of the gene. However, sequence comparisons of the first intron of AtDRTS2 and the second intron of AtDRTS1 could not detect common regions and a serial mutation analysis of the first AtDRTS2 intron could not identify regulatory elements enabling the meristematic activity of the gene. Nevertheless, the 5’UTR of AtDRTS2 is capable to confer strong meristematic expression when placed downstream of the AtDRS1 promoter suggesting an interchangeability between the two different introns and lack of specificity between promoter and intronic sequences. In addition, we found that the 5’UTR of AtDRTS2 acts at the transcriptional level and can confer meristematic activity also to the promoter of the AtBAM3 gene of A. thaliana, which is strongly and specifically expressed only in leaf tissues. This meristematic activating capacity, however, is not observed with a minimal 35S promoter which indicates that the first intron of AtDRTS2 cannot act autonomously as a promoter by itself but must interplay with additional regulatory regions.