RESUMO
Utilization of external succinate by Bacillus cereus and the properties of the purified succinate:menaquinone-7 reductase (SQR) were studied. Bacillus cereus cells showed a poor ability for the uptake of and respiratory utilization of exogenous succinate, thus suggesting that B. cereus lacks a specific succinate uptake system. Indeed, the genes coding for a succinate-fumarate transport system were missing from the genome database of B. cereus. Kinetic studies of membranes indicated that the reduction of menaquinone-7 is the rate-limiting step in succinate respiration. In accordance with its molecular characteristics, the purified SQR of B. cereus belongs to the type-B group of SQR enzymes, consisting of a 65-kDa flavoprotein (SdhA), a 29-kDa iron-sulphur protein (SdhB), and a 19-kDa subunit containing 2 b-type cytochromes (SdhC). In agreement with this, we could identify the 4 conserved histidines in the SdhC subunit predicted by the B. cereus genome database. Succinate reduced half of the cytochrome b content. Redox titrations of SQR-cytochrome b-557 detected 2 components with apparent midpoint potential values at pH 7.6 of 79 and -68 mV, respectively; the components were not spectrally distinguishable by their maximal absorption bands as those of Bacillus subtilis. The physiological properties and genome database analyses of B. cereus are consistent with the cereus group ancestor being an opportunistic pathogen.