text: The operon must respond to changes in the metabolic state of the cell. It is subject to growth rate dependent control, stringent control (control during the various stresses the cell is exposed to) and other forms of control.  Hence it stops biosynthesis if guanine can be obtained from the external medium, increases its expression if nucleotides are needed (for example during DNA replication) and balances the production of GMP with respect to AMP and the pyrimidine nucleotides.
  The branch point at IMP mentioned above is tightly controlled rigid node so as to have a balanced production of AMP and GMP. The gua operon is repressed by GMP and is induced by AMP. Similarly AMP synthesis is repressed by AMP itself while it is activated by GMP. This dual control ensures that there is a balance of flux between AMP and GMP and the flux partition remains relatively constant even in the face of perturbations. Some mechanisms by which this control is achieved are discussed below.