Nucleotide sequence reveals overlap between T4 phage genes encoding dihydrofolate reductase and thymidylate synthase

We have determined the nucleotide sequence of a 1075-base-pair HindIII fragment of the T4 phage genome. This fragment contains the structural gene (frd) for dihydrofolate reductase and part of the gene (td) encoding thymidylate synthase. The fragment contains a 579-base-pair open reading frame, encoding a 193-residue polypeptide with a calculated mass of 21,603 Da, in agreement with our reported subunit molecular mass of 23,000. The deduced amino acid sequence shows partial homology with other dihydrofolate reductases, with most of the identities lying in regions known to be involved in substrate binding and catalysis. The 3' end of the coding strand overlaps the coding region for thymidylate synthase; the sequence - ATGA -includes an opal terminator for the frd gene and an initiating triplet for the td gene. The deduced amino acid sequence from this initiating ATG is identical, for the first 20 residues, with the NH2-terminal 20 residues reported for the td protein (M. Belfort , A. Moelleken , G. F. Maley , and F. Maley (1983) J. Biol. Chem. 258, 2045-2051). The sequenced HindIII fragment was transferred into a high expression plasmid vector for large scale production of homogeneous T4 dihydrofolate reductase. The experimentally determined sequence of 20 residues at the NH2-terminus of this protein is identical with that deduced from the nucleotide sequence for T4 dihydrofolate reductase.