Kinetic and Regulatory Properties of Citrate Synthase from the Thermophilic Green Gliding Bacterium Chloroflexus Aurantiacus

Abstract

Chloroflexus aurantiacus is a photosynthetic member of a group of microbes which represent an extremely deep branching in the eubacterial line of descent (Gibson et al., 1985; Oyaizu et al., 1987). The other genera in this grouping, the green nonsulphur bacteria, are Herpetosiphon and Thermomicrobium, with which Chloroflexus shares little apparent phenotypic resemblance. Rlthough Chloroflexus contains BChl a and c, the latter localized in chlorosomes attached to the cytoplasmic membrane, 16S rRNA sequencing studies have shown no phylogenetic relatedness to the green sulphur bacteria (Gibson et al., 1985). Indeed, in terms of reaction centre photochemistry (Blankenship et al., 1983), some aspects of electron transport (Bruce et al., 1982) and the wide range of carbon sources utilized for photoheterotrophic or chemoheterotrophic growth (Madigan et al. f 1974), the metabolism of Chloroflexus most resembles that of the purple nonsulphur bacteria. Unlike the Rhodospirillaceae, however, the Calvin cycle is apparently not used to fix CO2 into cell material under autotrophic growth conditions (Holo and Sirevag, 1986) and a novel mechanism has been postulated. In addition, the thermophilic character of Chloroflexus (optimum growth at 55°C) is seemingly rare amongst phototrophic bacteria.