Sucrose Catabolism in Clostridium pasteurianum and Its Relation to N2 Fixation

摘要

The growth constant and Y (sucrose) (grams of cells per mole of sucrose) for NH₃-grown cultures of Clostridium pasteurianum were 1.7 times those of N₂-grown cultures, whereas the rate of sucrose utilized per gram of cells per hour was similar for both conditions. The Y (sucrose) of chemostat cultures grown on limiting NH₃ under argon at generation times equal to those of N₂-fixing cultures was less than that of cultures grown on excess NH₃, but cells of NH₃-limited cultures contained the N₂-fixing system in high concentration. The concentration of the N₂-fixing system in whole cells, when measured with adenosine triphosphate (ATP) nonlimiting, was more than twofold greater than the amount needed for the N₂ actually fixed. Thus, energy production from sucrose, and not the concentration of the N₂-fixing system nor the maximal rate at which N₂ could be fixed, was the limiting factor for growth of N₂-fixing cells. Either NH₃ or some product of NH₃ metabolism partially regulated the rate of sucrose metabolism since, when cultures fixing N₂, growing on NH₃, or growing on limiting NH₃ in the absence of N₂ were deprived of their nitrogen source, the rate of sucrose catabplism decreased. Calculations showed that the rate of ATP production was the growth rate-limiting factor in cells grown on N₂, and that the increased sucrose requirement of N₂-fixing cultures in part reflected the energy demand of N₂ fixation. Calculations indicated that whole cells require about 20 moles of ATP for the fixation of 1 mole of N₂ to 2 moles of NH₃.