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Homoprotocatechuate (HPC; 3,4-dihydroxyphenylacetate) is catabolized to Krebs cycle intermediates via extradiol (meta-) cleavage and the necessary enzymes are chromosomally encoded in a variety of bacteria. Based on an analysis of the cloned pathway genes, the Escherichia coli C hpc gene cluster was thought to be arranged in two gene blocks transcribed from a central, divergent, operator/promoter region, which was negatively regulated by the Hpc repressor. By a variety of techniques including expression of cloned hpc genes in pUC18/19 vectors, unidirectional deletion subcloning, hybridization studies and nucleotide sequencing it has now been shown that the hpc pathway structural genes are transcribed in one direction. These experiments have also indicated that a decarboxylase and an isomerase of the pathway are encoded by a single gene (hpcE) and have established the exact structural …