IDENTIFICATION OF THE L-TARTRATE DEHYDRATASE GENES (TTDA AND TTDB) OF ESCHERICHIA-COLI AND EVOLUTIONARY RELATIONSHIP WITH THE CLASS-I FUMARASE GENES

The genes encoding an oxygen-labile stereospecific L-tartrate dehydratase (L-Ttd, EC 4.2.1.32) have been identified as the orfZ1 and orfZ2 genes located upstream of the rpsU-dnaG-rpoD operon at 67 min in the Escherichia coli linkage map. They were previously cloned and sequenced by M. Nesin and others (Gene 51, 149-161, 1987) and have now been independently cloned, partially resequenced, and designated as an operon (ttdAB) containing two translationally coupled genes. The enzyme behaves as a tetramer (M(r) 105000) containing two pairs of non-identical subunits, TtdA (M(r) 32589) and TtdB (M(r) 22641), which otherwise resembles the homodimeric iron-sulphur-containing Class I fumarases of E. coli and Bacillus stearothermophilus. The amino acid sequences of the TtdA-TtdB subunits are colinearly related to a single fumarase subunit, indicating a common evolutionary ancestry. E. coli can use L-, D- and meso-tartrates as aerobic growth substrates and as reducible substrates for supporting anaerobic growth on glycerol. L-Ttd was induced during anaerobic growth on glycerol plus L- and meso-tartrates, and a stereospecific D-tartrate dehydratase was induced by all three stereoisomers under comparable conditions. No meso-tartrate dehydratase was detected, nor were any dehydratases detected after aerobic growth on tartrate minimal media suggesting that different catabolic routes operate under aerobic conditions.