Negative and Positive Controls of Transcription

The system of regulation in lactose and tryptophan operons outlined above is essentially a negative control in the sense that the operon is normally 'on' but is kept 'off' by the regulator gene. In other words, the gene is not allowed to express unless required. However, it is shown that through cga protein (or CAP) and cyclic AMP, a positive control is also exercised in lac operon (Fig. 35.10).

In a positive control the regulator gene will stimulate the production of enzyme as in case of arabinose operon of E. coll. The gene without the activator will be inactive in this case (Fig. 35.18). Most systems on regulation, studied earlier, used a negative control. The positive control systems were studied only later.

In arabinose operon (also known as OBAD operon) shown in Figure 35.18, O, B and A are structural genes, i is the initiating site, O is the operator site and C is a gene comparable to regulator gene of 'lac' operon. Through the use of two mutations C^- (which is characterized by reduction in transcription) and C^c (which is characterized by the constitutive synthesis of enzymes), it could be shown that the gene C gives rise to a protein P₁ which works as a repressor in the absence of arabinose. In the presence of arabinose, P₁ is converted into P₂, a protein working as a stimulator in transcription. The role of P₁ as repressor was confirmed when it was observed that araC⁺ is dominant over araC^c, because if P₁is not a repressor, then araC^c should have been dominant over araC⁺. In this manner, arabinose operon, which was once thought to have only positive control, is now known to have both a positive and a negative control system.