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Fermentative growth via the arginine deiminase pathway is mediated by the enzymes arginine deiminase, carbamate kinase, and catabolic ornithine transcarbamylase and by a membrane-bound arginine-ornithine antiporter. Recently we reported the characterization of catabolic ornithine transcarbamylase and the corresponding gene, arcB, from Halobacterium salinarium (formerly Halobacterium halobium). Upstream of the arcB gene, three additional open reading frames with halobacterial codon usage were found. They were identified as the arcC gene coding for carbamate kinase, the arcA gene coding for arginine deiminase, and a gene, tentatively termed arcR, coding for a putative regulatory protein. The identification of the arcC and arcA genes was verified, respectively, by heterologous expression of the enzyme in Haloferax volcanii and by protein isolation and N-terminal sequence determination of three peptides. The gene order arcRACB differs from the gene order arcDABC in Pseudomonas aeruginosa, the only other organism for which sequence information is available. Transcripts from H. salinarium cultures grown fermentatively or aerobically were characterized by Northern (RNA) blot and primer extension analyses. It was determined (i) that monocistronic transcripts corresponding to the four open reading frames exist and that there are three polycistronic transcripts, (ii) that the level of induction during fermentative growth differs for the various transcripts, and (iii) that upstream of the putative transcriptional start sites for the three structural genes there are sequences with similarities to the halobacterial consensus promoter. The data indicate that expression of the arc gene cluster and its regulation differ in H. salinarium and P. aeruginosa.