Chloroaromatics, a major class of industrial pollutants, may be oxidatively metabolized to chlorocatechols by soil and water microorganisms that have evolved catabolic activities toward these xenobiotics. We show here that 4-chlorocatechol can be further transformed by enzymes of the ubiquitous 3-oxoadipate pathway. However, whereas chloromuconate cycloisomerases catalyze the dechlorination of 3-chloro-cis,cis-muconate to form cis-dienelactone, muconate cycloisomerases catalyze a novel reaction, i.e. the dechlorination and concomitant decarboxylation to form 4-methylenebut-2-en-4-olide (protoanemonin), an ordinarily plant-derived antibiotic that is toxic to microorganisms.