Reversible inactivation of isocitrate dehydrogenase in Escherichia coli.

Escherichia coli can grow aerobically in defined media containing inorganic salts and any one of a wide range of carbon compounds. Whatever the nature of the carbon source. it is fed into the amphibolic pathways [glycolysis, pentose phosphate pathway, Krebs (tricarboxylic acid) cyclel, from which stem all the biosynthetic pathways required for making the components of new cell material. The operation of the amphibolic pathways also traps energy in the forms (ATP, NADPH, H+) required for biosynthesis and growth. These processes are common to growth on any carbon source, but biosynthesis also relies on anaplerotic sequences of metabolism which replenish the pools of some compounds (e.g. in the Krebs cycle) which are the starting points for biosynthesis (Kornberg. 1966). Different anaplerotic sequences are used according to the point at which any particular carbon source enters the amphibolic pathways. For example, growth on glucose (Fig. 1) depends on the