Regulation of the Escherichia coli dapD gene involved in diaminopimelate and lysine biosynthesis was unknown as no convenient enzymatic assay was available until recently. This gene was cloned into pBR322 from a lambda transducing phage; its complete nucleotide sequence was established. This sequence shows that the dapD gene is composed of a single cistron encoding a 274-amino acid polypeptide, Mr 30,040. Enzymatic activity measurements show that this gene encodes the tetrahydrodipicolinate N-succinyltransferase which catalyzes the third step of the specific lysine-diaminopimelate pathway. The transcriptional start of the dapD gene was localized; the identified promoter signals are weak compared to those from the E. coli promoter consensus sequence. The dapD gene-coding sequence is followed by a typical rho-independent transcriptional termination sequence. A study using an operon fusion constructed in vitro between the dapD promoter and the galK structural gene indicated that dapD gene expression is repressed by lysine; no attenuation-like sequence can be found to account for this regulation. At the present time, out of the 9 genes involved in diaminopimelate and lysine biosynthesis, 6 are known to be lysine regulated.