Why does the Gene 1 mutant remain prototrophic and nitrogen-fixing?

Learning Goal: I’m working on a biology multi-part question and need an explanation and answer to help me learn.

Representative abundances of selected metabolites found in a small molecule extract (SME) from your designated wild-type bacterial strain are shown in the table below. Note: These reported abundances are dependent on the solubility and ionization characteristics of each compound within this complex mixture. They, therefore, have no direct connection with the metabolite concentrations in the cell.

A number of mutants with a defective TCA cycle enzyme have been isolated. You prepared a small molecule extract from ONE mutant that has been identified in the gene encoding the enzyme malate dehydrogenase.

Q2.3. Identify the metabolites expected to change drastically in the mutant and indicate whether you expect the concentration to increase or decrease (3 marks).

The IMG search showed that there are two genes that code for a specific TCA cycle enzyme in this strain. You used a targeted method to create a point mutation in each gene. The phenotypes of the mutant are presented in the following table.

Q2.4. Why does the Gene 1 mutant remain prototrophic and nitrogen-fixing? (3 marks)

Q2.5. Why does the Gene 2 mutant become an auxotroph and also lose nitrogen-fixing ability? (3 marks)

Q2.6. Can you predict the conditions/environment where (a) gene 1, and (b) gene 2 are expressed? (3 marks)