Q.59 Lysine is being produced in a lab-scale reactor by a threonine auxotroph. After 2 weeks of
operation it was observed that the concentration of lysine in the reactor was gradually decreasing.
Microbiological assays of reactor samples showed absence of contamination and recorded data
showed no change in the operating conditions. The most probable reason for decrease in lysine
concentration may be attributed to
(A) accumulation of ethanol (B) growth of revertants
(C) production of citric acid (D) unutilized phosphoenol pyruvate
Threonine auxotrophs overproduce lysine due to blocked threonine synthesis, diverting aspartate pathway flux toward lysine. After 2 weeks, lysine levels decrease without contamination or changed conditions because revertants emerge. The correct answer is (B) growth of revertants.
Correct Answer
Growth of revertants causes the lysine drop. These mutants regain wild-type function, resuming threonine synthesis and feedback inhibition of aspartate kinase, halting lysine overproduction.
Option Explanations
| Option | Explanation |
|---|---|
| (A) Accumulation of ethanol | Ethanol buildup relates to overflow metabolism in yeasts under anaerobic conditions, not lysine fermentation by bacterial auxotrophs. No link to lysine decrease exists here. |
| (B) Growth of revertants | Correct. Revertants reverse the auxotrophic mutation over time, restoring threonine production. This inhibits lysine synthesis via feedback on aspartokinase. Common in long-term lab cultures. |
| (C) Production of citric acid | Citric acid overproduction occurs in Aspergillus under specific pH/glucose conditions, unrelated to amino acid auxotrophs or lysine pathways. |
| (D) Unutilized phosphoenol pyruvate | PEP feeds aspartate synthesis but remains available; unused PEP would boost, not reduce, lysine. Auxotrophy blocks downstream, not PEP utilization. |
Fermentation Implications
Revertants highlight genetic instability in auxotrophic strains during extended batch runs. Regular subculturing or chemostat dilution prevents dominance. Biotech processes favor stable engineered strains like deregulated aspartokinase mutants in Corynebacterium glutamicum.
