When both glucose and lactose are present in the medium, the transcription of the genes z, y and a are inhibited. This phenomenon is called catabolite repression or glucose effect.
The effect of glucose is mediated by cAMP and CRP. Glucose and cAMP concentrations are reciprocally related.
In the presence of high glucose, irrespective of lactose concentration, less cAMP is formed, consequently leading to less cAMP-CRP complex. The CRP binding site is free.
This consequence leads to an inhibition of expression of the structural genes of the lac operon. Conversely, low glucose concentration is complemented by high lactose concentration. The CRP and The operator sites are occupied by cAMP-CRP complex and RNA polymerase, respectively.
This results in an augmented expression. Low glucose and low lactose concentrations inhibit expression. Thus, it is clear that cAMP-CRP is a positive regulatory factor, while the repressor is negative. CAMP and CRP co-ordinate several operons, primarily involved in the