SEQUENCE OF THE ESCHERICHIA-COLI C-HOMOPROTOCATECHUIC ACID DEGRADATIVE OPERON COMPLETED WITH THAT OF THE 2,4-DIHYDROXYHEPT-2-ENE-1,7-DIOIC ACID ALDOLASE-ENCODING GENE (HPCH)

The homoprotocatechuic acid (HPC) pathway is a typical catabolic sequence for converting peripheral metabolites into intermediates of central metabolism. How the pathway enzymes that catalyse such natural sequences have arisen is as yet uncertain, but the explanation is likely to be of interest in devising pathways to catabolise the man-made chemicals that are increasingly found in the environment. The nucleotide (nt) sequence of the Escherichia coli C 2,4-dihydroxyhept-2-ene-1,7-dioic acid (HHED) aldolase-encoding gene (hpcH) reported here completes the sequencing of the HPC pathway genes, and so makes it possible to assess the relatedness of all the pathway enzymes. There were no striking amino acid (aa) sequence identities between any of the pathway enzymes, suggesting that they had not arisen by duplication of an ancestral gene, with subsequent divergence. The HHED aldolase showed no striking identity (16-22%) with the aldolases from five other bacteria catalysing the analogous reaction in the catechol meta-fission pathway. However, there was significant aa identity (47.8%) with an E. coli K-12 open reading frame (ORF) of as yet unknown function, suggesting that this ORF may encode an aldolase of some kind.