Phenylacetic acid degradation (PAD) proteins PaaZ (Escherichia coli) and PaaN (Pseudomonas putida) are putative aromatic ring cleavage enzymes of the aerobic PA catabolic pathway. PaaZ mutants were defective for growth with PA as a sole carbon source due to interruption of the putative ring opening system. This CD is limited to bacterial monofunctional enzymes.