Two alternative forms of protocatechuate 3,4-dioxygenase (PCase) have been purified from Moraxella sp. strain GU2, a bacterium that is able to grow on guaiacol or various other phenolic compounds as the sole source of carbon and energy. One of these forms (PCase-P) was induced by protocatechuate and had an apparent molecular weight of 220,000. The second form (PCase-G) was induced by guaiacol or other phenolic compounds, such as 2-ethoxyphenol or 4-hydroxybenzoate. It appeared to be smaller (Mr

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... 158,000), and its turnover number was about double that of the former enzyme. Both dioxygenases had similar properties and were built from the association of equal amounts of nonidentical subunits, a and j, which were estimated to have molecular weights of 29,500 and 25,500, respectively. The (al)3 and (aP)4 structures were suggested for PCases G and P, respectively. On the basis of two-dimensional gel electrophoresis, the a and polypeptides of PCase-G differed from those of PCase-P. Amino acid analysis supported this conclusion. Both PCases, however, had several other properties in common. It is proposed that both isoenzymes were generated from different sets of a and P subunits, and the significance of these data is discussed.