Mouse thymidylate synthase messenger RNA lacks a 3' untranslated region

Analysis of the sequence of cDNA corresponding to mouse thymidylate synthase (5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1.45) mRNA revealed that the termination codon TAA was followed immediately by a poly(A) sequence. This raised the possibility that mouse thymidylate synthase mRNA lacks a 3' untranslated region. In the present study, we have further investigated this possibility. DNA corresponding to the 3' end of the thymidylate synthase gene was isolated from a genomic library. The sequence of the genomic DNA was identical to that of the cDNA in the coding region. However, the termination codon was TAG in the genomic sequence rather than TAA, and poly(A) was not present in the genomic DNA. Sequences flanking the site of poly(A) addition were in good agreement with polyadenylylation consensus sequences. S1 nuclease analysis revealed that approximately 80% of the thymidylate synthase mRNA molecules were polyadenylylated at the termination codon. A secondary polyadenylylation site was detected 190-200 nucleotides downstream of the primary site. We conclude that the major species of mouse thymidylate synthase mRNA lacks a 3' untranslated region and that the final A of the termination codon is added by poly(A) polymerase. It appears that a 3' untranslated region is not essential for the accumulation or translation of this mRNA.