The complete nucleotide sequence of the asd gene of Streptococcus mutans encoding aspartate beta-semialdehyde dehydrogenase (EC 1.2.1.11), an enzyme comprised of 357 amino acids, having an Mr of 38,897 and active in the biosynthetic pathway of lysine, threonine, methionine, diaminopimelic acid, and isoleucine, has been determined. In addition we report the 276 nucleotides upstream of the structural gene which contain a highly efficient promoter identified by both RNA polymerase binding and in vitro transcription analysis. A leader transcript which terminates at a fixed point immediately preceding the asd promoter region was identified in the DNA sequence and confirmed by in vitro transcription analysis as well. The close proximity of this transcript and its p-independent transcriptional terminator to the asd coding sequence suggests involvement in a mechanism of regulation. Message stability experiments indicate the half-life of asd specific messages to be comparable to that of Escherichia coli messages. Conditions of varying concentrations of lysine, threonine, and methionine exert no apparent control over expression of the S. mutans asd gene in Escherichia coli suggesting the requirement of an accessory regulatory element specific for the S. mutans asd gene.