Histidine regulation in Salmonella typhimurium: an activator attenuator model of gene regulation

An activator attenuator model of positive control, as opposed to the classic repressor operator model of negative control, is proposed for the major operon specific mechanism governing expression of the histidine gene cluster of S. typhimurium. Evidence for this mechanism is derived from experiments performed with a coupled in vitro transcription translation system, as well as with a minimal in vitro transcription system. The product (G enzyme, or N 1 [5' phosphoribosyl] adenosine triphosphate:pyrophosphate phosphoribosyl transferase; EC 2.4.2.17) of the first structural gene (hisG) of the histidine operon is not involved in the positive control mechanism. However, a possible role for G enzyme as an accessory negative control element interacting at the attenuator can be accommodated in our model. The operon specific mechanism works in conjunction with an independent mechanism involving guanosine 5' diphosphate 3' diphosphate (ppGpp) which appears to be a positive effector involved in regulating amino acid producing systems in general.