In Pseudomonas aeruginosa, arginine catabolism via the arginine deiminase pathway depends on the anaerobically inducible arcDABC operon, whose expression is further modulated by mRNA processing. Fusion of the cloned arc operon to an external tac promoter did not alter the processing pattern in P. aeruginosa and allowed heterologous expression in Escherichia coli. Processing within a specific region of the arcD mRNA was similar in P. aeruginosa and in E. coli. In E. coli, a conditional temperature-sensitive (ts) mutation in the gene specifying RNase E prevented cleavage of the arc mRNA at the non-permissive temperature, whereas mutations in the genes encoding RNase III or RNase P had no effect. We therefore speculate that in P. aeruginosa, an RNase E-like enzyme exists which is involved in the specific processing of the arc mRNA.