The nucleotide sequence of the gene for folylpoly-gamma-glutamate synthetase-dihydrofolate synthetase (folC) has been determined. The deduced amino acid sequence codes for a protein of Mr 45,380 and contains regions with homology to the A and B regions of ATP-binding sites. The folC gene is adjacent to a gene located 70 base pairs upstream of the initiation codon for folC. The nucleotide sequence of this upstream gene was also determined. Deletion of the upstream gene sequences from recombinant plasmids containing the folC gene results in about a 100-fold decrease in plasmid-dependent folylpolyglutamate synthetase activity, suggesting that the major promoter for folC is 5' to the upstream gene. The upstream gene codes for a protein of Mr 33,346, which is expressed in maxicells and amplified in cells containing the upstream gene in recombinant pUC8 plasmids. Expression of the upstream gene in maxicells was greater than that of folC, as determined by the intensity of 35S-labeled proteins after sodium dodecyl sulfate-gel electrophoresis. A region of dyad symmetry exists between the coding sequences of the two genes which may code for a transcription termination signal and be responsible for the attenuation of the expression of the folC gene relative to the upstream gene. The folC gene is located about 1 kilobase upstream of the purF gene region at 50 min on the Escherichia coli map. The function of the upstream gene product is unknown. It contains sequences with homology to metal-binding domains in nucleic acid-binding proteins. A new purification procedure for obtaining large quantities of folylpolyglutamate synthetase-dihydrofolate synthetase is described.