The genes that encode the α and β subunits of protocatechuate 3,4- dioxygenase (3,4-PCD [EC 220.127.116.11]) were cloned from a Pseudomonas putida (formerly P. aeruginosa) (ATCC 23975) genomic library prepared in λ phage. Plaques were screened by hybridization with degenerate oligonucleotides designed using known amino acid sequences. A 1.5-kb SmaI fragment from a 15- kb primary clone was subcloned, sequenced, and shown to contain two successive open reading frames, designated pcaH and pcaG, corresponding to the β and α subunits, respectively, of 3,4-PCD. The amino acid sequences deduced from pcaHG matched the chemically determined sequence of 3,4-PCD in all except three positions. Cloning of pcaHG into broad-host-range expression vector pKMY319 allowed high levels of expression in P. putida strains, as well as in Proteus mirabilis after specific induction of the plasmid-encoded nahG promoter with salicylate. The recombinant enzyme was purified and crystallized from P. mirabilis, which lacks an endogenous 3,4-PCD. The physical, spectroscopic, and kinetic properties of the recombinant enzyme were indistinguishable from those of the wild-type enzyme. Moreover, the same transient enzyme intermediates were formed during the catalytic cycle. These studies establish the methodology which will allow mechanistic investigations to be pursued through site-directed mutagenesis of P. putida 3,4-PCD, the only aromatic ring-cleaving dioxygenase for which the three-dimensional structure is known.