Summary: The genes encoding an oxygen-labile stereospecific L-tartrate dehydratase (L-Ttd, EC have been identified as the and genes located upstream of the operon at 67 min in the linkage map. They were previously cloned and sequenced by M. Nesin and others ( 51, 149-161, 1987) and have now been independently cloned, partially resequenced, and designated as an operon () containing two translationally coupled genes. The enzyme behaves as a tetramer (Mr 105000) containing two pairs of non-identical subunits, TtdA (M 32589) and TtdB (M 22641), which otherwise resembles the homodimerie iron-sulphur-containing Class I fumarases of and The amino acid sequences of the TtdA-TtdB subunits are colinearly related to a single fumarase subunit, indicating a common evolutionary ancestry. can use L-, D- and -tsrtrates 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 -tartrates, and a stereospecific D-tartrate dehydratase was induced by all three stereoisomers under comparable conditions. No -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.


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