Sucrose Catabolism in Clostridium pasteurianum and Its Relation to N 2 Fixation

Abstract

The growth constant and Y (sucrose) (grams of cells per mole of sucrose) for NH 3 -grown cultures of Clostridium pasteurianum were 1.7 times those of N 2 -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 NH 3 under argon at generation times equal to those of N 2 -fixing cultures was less than that of cultures grown on excess NH 3 , but cells of NH 3 -limited cultures contained the N 2 -fixing system in high concentration. The concentration of the N 2 -fixing system in whole cells, when measured with adenosine triphosphate (ATP) nonlimiting, was more than twofold greater than the amount needed for the N 2 actually fixed. Thus, energy production from sucrose, and not the concentration of the N 2 -fixing system nor the maximal rate at which N 2 could be fixed, was the limiting factor for growth of N 2 -fixing cells. Either NH 3 or some product of NH 3 metabolism partially regulated the rate of sucrose metabolism since, when cultures fixing N 2 , growing on NH 3 , or growing on limiting NH 3 in the absence of N 2 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 N 2 , and that the increased sucrose requirement of N 2 -fixing cultures in part reflected the energy demand of N 2 fixation. Calculations indicated that whole cells require about 20 moles of ATP for the fixation of 1 mole of N 2 to 2 moles of NH 3 .