Mutants defective in isomerase and decarboxylase activities of the 4-hydroxyphenylacetic acid meta-cleavage pathway in Pseudomonas putida

Abstract

Degradation of 2-hydroxy-5-carboxymethylmuconic semialdehyde, the ring fission product of the 4-hydroxyphenylacetate meta-cleavage pathway, by mutant strains P23X19 and P23X16 of Pseudomonas putida NCI B 9865 was studied. Both mutants were unable to grow on either 4-hydroxyphenylacetate of 3,4-dihydroxyphenylacetate. Cell extracts of P23X19, grown in the presence of 3,4-dihydroxyphenylacetate, degraded the ring fission product to a compound that accumulated and had maximum UV absorption at 300 nm, pH 7.4, and 345 nm, pH 12. These are the spectral characteristics of 2-keto-5-carboxymethylhex-3-ene-1,6-dioate, the substrate for the decarboxylase in this pathway. This observation is consistent with P23X19's being decarboxylase defective. Cell extracts of P23X16, grown in the presence of 3,4-dihydroxyphenylacetate, degraded the ring fission product to a compound that accumulated and has maximum UV absorption at 295 nm, pH 7.4, and 345 nm, pH 12. This compound spontaneously degraded to a compound with the spectral properties of the decarboxylase substrate. The compound accumulated by P23X16 was also obtained when the decarboxylase substrate was treated with borate. It is suggested that the compound accumulated by P23X16 is the substrate of an isomerase. The results are consistent with P23X16's being unable to synthesize a functional isomerase while retaining decarboxylase activity and establish the physiological importance of an enzyme-catalyzed isomerization in the meta-cleavage degradation of 4-hydroxyphenylacetate.