Although Enteroccus faecalis is the paradigm for biochemical studies on the arginine deiminase (ADI) pathway of fermentative arginine catabolism, little genetic information exists on this pathway in this organism. We fill this important gap by characterizing, in an 8,228-bp region cloned from a gt11 genomic library of E. faecalis, a five-gene cluster forming a transcriptional unit (revealed by Northern blots and primer extension in E. faecalis) that corresponds to the ADI operon. Four additional genes in the opposite DNA strand and one in the same DNA strand are also identified. Studies on the protein products, including heterologous expression and/or sequence comparisons, allow us to ascertain or propose functions for all but 1 of the 10 genes. The ADI operon genes, arcABCRD, encode, respectively, ADI, ornithine transcarbamylase, carbamate kinase, a putative Crp/Fnr-type regulator (ArcR), and a putative ornithine-arginine antiporter (ArcD). Arginine induces the expression of arcABCRD, most likely by means of two homologous ArgR/AhrC-type regulators encoded by two genes, argR1 and argR2, that precede arcABCRD in each DNA strand and that are transcribed monocistronically, their transcription being influenced differentially by glucose and arginine. Potential ArgR1/ArgR2 (double and single) binding sequences are found in the promoter regions of arcA and of argR1/argR2 themselves. In addition, putative binding sequences for ArcR and for CcpA are found, respectively, in the argR1/ argR2 and arcA promoter regions. Of the three other genes identified, two form a transcriptional unit and encode a putative metal-sensitive transcriptional regulator (ArsR) and a cysteine protease.A number of microorganisms generate ATP fermentatively from arginine by a route (1) (Fig. 1) in which arginine is deiminated by arginine deiminase (ADI; EC 3.5.3.6), the resulting citrulline is phosphorolyzed by ornithine transcarbamylase (OTC; EC 2.1.3.3), giving ornithine and carbamoyl phosphate, and finally the carbamoyl phosphate is used to phosphorylate ADP in a reaction catalyzed by carbamate kinase (CK; EC 2.7.2.2), yielding ATP. This route, called the ADI pathway, operates in many eubacteria (1,10,12,64) and in some archaea (53) and unicellular eukaryotes (59, 71), but it has not been found in multicellular organisms.Possibly no microbial species has been more important for the biochemical characterization of the ADI pathway than Enterococcus faecalis (formerly Streptococcus faecalis). The ADI pathway was originally demonstrated and characterized in this microorganism (22,25,62), and its enzymes were purified (40, 41, 50) and shown to be coordinately induced by arginine (61). Recently the first three-dimensional structure of a CK was determined (38) on the enzyme isolated from what was believed to be Enterococcus faecium (6) but that we presently show to be E. faecalis (called here E. faecalis SD10).Despite the abundant information derived from E. faecalis on the biochemistry of the ADI pathway, there was no other genetic information about...