Mapping and expression of a bifunctional thymidylate synthase, dihydrofolate reductase gene from maize

A bifunctional gene (ZmDHFR-TS) encoding dihydrofolate reductase (DHFR) and thymidylate synthase (TS) was cloned from a Zea mays cDNA library. Both of these enzymes are involved in nucleotide biosynthesis, specifically in the formation of thymidine monophosphate (TMP). Comparison of the deduced amin o acid sequence with DHFR-TS sequences from three other plant sources revea led over 75% similarity and motifs typical of DHFR-TS proteins. Two copies of the gene were mapped to chromosomes 2 and 4. This represents the first D HFR-TS gene cloned from a monocotyledonous plant. Expression of ZmDHFR-TS w as examined in developing kernels and various tissues of maize by RNA gel b lot hybridization analysis in order to determine the relationship between e xpression of this gene and DNA synthesis. RNA transcripts for ZmDHFR-TS acc umulated to high levels in developing maize kernels when endosperm cells we re undergoing endoreduplication and cell division. Meristematic maize tissu es had high levels of ZmDHFR-TS mRNA, but transcripts were barely detectabl e in RNA isolated from the root elongation zone and from mature leaf tissue s.