The lactate utilizing strain of Selenomonas ruminantium 5934e was found to contain three lactate dehydrogenase (LDH) activities in sonicated cell extracts. One activity, an NAD dependent L-LDH ...(L-nLDH) was measured at 15-fold greater levels in extracts of cells grown to mid-exponential phase on glucose compared to cells grown to the equivalent growth stage on DL-lactate. A second nLDH activity specific for D-lactate (D-nLDH) was detected to similar levels in both lactate-grown cell extracts and glucose-grown cell extracts. The third activity, an NAD independent DLDH (D-iLDH) was very low in cells grown on glucose but was induced more than 10-fold when DL-lactate was used as the carbon source. The three LDH activities could be separated by gel filtration. Recovery of the activities was low due to the apparent instability of the enzymes at 4 degrees C which was most pronounced in the case of the D-iLDH. A Km for lactate of 0.5 mM was stimulated for the D-iLDH and this was considerably lower than the values of 45 mM and 70 mM measured for L-nLDH and D-nLDH respectively. It is proposed that the D-iLDH may be largely responsible for the formation of pyruvate in lactate-grown cells of S. ruminantium strain 5934e. Three other lactate utilizing strains of S. ruminantium, HD4, 5521C1 and JW13 exhibited a similar profile of LDH activities to strain 5934e when grown on glucose and DL-lactate.
The utilization of sucrose by Clostridium acetobutylicum ATCC 824 was investigated. Sucrose was found to be transported via a phosphoenol-pyruvate (PEP)-dependent phosphotransferase system (PTS) and ...a metabolic pathway identical to that previously identified in C. beijerinckii, was established. The genes encoding the proteins of this pathway were identified from the C. acetobutylicum genome sequence, in the order scrAKB encoding Enzyme II of the sucrose PTS, fructokinase and sucrose 6-phosphate hydrolase respectively. While the pathway for sucrose metabolism is conserved between C. acetobutylicum and C. beijerinckii, the operons show considerable differences in organization and regulatory elements. The C. acetobutylicum scr operon contains the elements of an antiterminator-mediated regulation mechanism, typical of the BgIG family of regulators. The scrT gene, located upstream of scrA encodes an antiterminator that is preceded by a transcription terminator, which is overlapped by a classical ribonucleic antiterminator (RAT) sequence. We also propose the existence of a new variant RAT-like sequence which overlaps a terminator between scrT and the downstream structural genes.
Extracts prepared from cultures of Bacillus subtilis, grown on maltose as the sole carbon source, lacked maltose phosphotransferase system activity. There was, however, evidence for a maltose ...phosphorylase activity, and such extracts also possessed both glucokinase and glucose phosphotransferase system activities. Maltose was accumulated by whole cells of B. subtilis by an energy-dependent mechanism. This uptake was sensitive to the effects of uncouplers, suggesting a role for the proton-motive force in maltose transport. Accumulation of maltose was inhibited in the presence of glucose, and there was no accumulation of maltose by a strain carrying the ptsI6 null-mutation. A strain carrying the temperature-sensitive ptsI1 mutation accumulated maltose normally at 37 degrees C but, in contrast to the wild-type, was devoid of maltose transport activity at 47 degrees C. The results indicate a role for the phosphotransferase system in the regulation of maltose transport activity in this organism.
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