Structure of the gene for mouse thymidylate synthase. Locations of introns and multiple transcriptional start sites

We have isolated and analyzed the structure of the gene for thymidylate synthase from a 5-fluoro-2'-deoxyuridine-resistant 3T6 cell line that overproduces thymidylate synthase 50-fold by virtue of gene amplification. Three overlapping DNA segments containing the entire thymidylate synthase gene were identified in Charon 35 genomic libraries. Sequence analysis revealed that all of the coding regions were contained in a 12-kilobase segment of DNA. The gene has 6 introns ranging from 0.6 to 4.1 kilobases in length. The sequences at the 5' and 3' ends of each intron conformed to the consensus sequences for mammalian introns. S1 nuclease and primer extension assays showed that transcription of the thymidylate synthase gene initiates at several sites within a 66-nucleotide region. There are no TATAA or CCAAT sequences in the vicinity of the initiation sites. However, the region does contain DNA sequences that are known to stimulate binding of the transcription factors Sp1 and USF. These binding sites are adjacent to each other, suggesting that the two proteins may bind to the upstream region of the thymidylate synthase gene in a cooperative or competitive manner.