Hypotheses based on data and literature:

• ¹⁵N from the medium is used preferentially for cyanophycin synthesis unless source is blocked (starvation, no ammonium permease, or no nitate reductase), in which case cells will break down 14N proteins to make cyanophycin
  
  
• Two enzymes -- ammonium permease and nitrate reductase -- are necessary for utilization of the two N sources; ammonium permease will be fully expressed only when cells are starved, nitrate reductase only when cells are starved or only nitrate is present. When CM is added, neither protein can be made.