Operon structure and nucleotide homology of the chlorocatechol oxidation genes of plasmids pJP4 and pAC27

Abstract

Alcaligenes eutrophus harboring plasmid pJP4 (strain JMP134) is capable of growing on both 2,4-dichlorophenoxyacetate (2,4-D) and 3-chlorobenzoate (3-Cba), while Pseudomonas putida carrying plasmid pAC27 (strain AC867) can utilize only 3-Cba as the sole carbon source. The tfdCDEF operon of the pJP4 plasmid and the clcABD operon of plasmid pAC27 each encode enzymes for the degradation of chlorocatechols (Clc), key intermediates in the catabolism of 2,4-D and 3-Cba. Similarities in the nucleotide (nt) sequences of genes tfdC and clcA, encoding pyrocatechases, were reported earlier [Ghosal and You, Mol. Gen. Genet. 211 (1988a) 113-120]. Genes tfdD and clcB, encoding Clc-specific cycloisomerases, have been completely sequenced. The tfdD gene (1107 bp) is slightly smaller than gene clcB (1113 bp). Comparison of the two cycloisomerase-encoding genes reveals that the nt sequences are 63% homologous with 62% homology in the deduced amino acid (aa) sequences of the polypeptides they encode. Genes tfdD and tfdE are contiguous in the tfdCDEF operon, whereas the corresponding genes, clcB and clcD, of the clcABD operon, are known to be separated by a long open reading frame of unknown function. The predicted N-terminal aa sequences of the two hydrolase-encoding genes, tfdE and clcD, also show homology. The structural and nt homologies between the two Clc operons, tfdCDEF and clcABD, suggest their relatedness.