Fig. 1.

Model of the core interactions in the lactose system. The pools of nucleotides and amino acids are assumed to be constant and are not shown with explicit variables. The polycistronic messenger RNA encodes proteins that include a permease for the transport of extracellular lactose into the cell, the enzyme β-galactosidase that catalyzes both the synthesis and degradation of the inducer allolactose, as well as a transacetylase that is not represented. The regulator is a repressor of transcription and its level is constitutively expressed. Filled arrows represent interactions that are positive, open arrows those that are negative.