SUMMARY: Three sharply distinct phases were found in the growth of in a continuous fermenter with 100% cell recycle. They were: an initial phase of batch-type growth reaching exponential rates that terminated abruptly when the glucose concentration in the fermenter fell below the cell's transport threshold; a second phase of linear growth whose rate was dependent on the rate of glucose provision to the fermenter, but whose length was fixed and independent of growth rate; and a third phase of slower linear growth, whose rate was also dependent on the glucose provision rate, characterized by a restriction in RNA accumulation, and indefinite in length. Guanosine 5′-diphosphate 3′-diphosphate (ppGpp) accumulation commenced at the start of phase 2 and rose throughout the phase. It reached a maximum at the start of phase 3, concurrent with the curbing of RNA accumulation; thereafter, the ppGpp concentration was constant unless the rate of glucose supply was increased, at which point it fell to a phase 2 level and the cells returned to a corresponding phase 2 growth rate. The length of growth phase 2 was changed in a precise manner by mutations in the and loci. Phase 2 was longer for and mutants (decreased capability for ppGpp synthesis) and shorter for mutants (decreased capability for ppGpp degradation). It is concluded that the continuously falling energy supply per cell in a recycling fermenter invokes ppGpp accumulation by at least three biochemical mechanisms and that the third, final growth phase in the fermenter is an indefinitely extended manifestation of strong regulation by ppGpp.


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