Summary of lac Operon responses
| Glucose | Lactose | CAP binds | Repressor binds | Level of transcription |
| + | – | – | + | NIL |
| + | + | – | – | very low |
| – | – | + | + | NIL |
| – | + | + | – | Strong |
- Glucose present, lactose absent: No transcription of the lac operon occurs. That’s because the lac repressor remains bound to the operator and prevents transcription by RNA polymerase. Also, cAMP levels are low because glucose levels are high, so CAP is inactive and cannot bind DNA.
- Glucose present, lactose present: Low-level transcription of the lac operon occurs. The lac repressor is released from the operator because the inducer (allolactose) is present. cAMP levels, however, are low because glucose is present. Thus, CAP remains inactive and cannot bind to DNA, so transcription only occurs at a low, leaky level.
- Glucose absent, lactose absent: No transcription of the lac operon occurs. cAMP levels are high because glucose levels are low, so CAP is active and will be bound to the DNA. However, the lac repressor will also be bound to the operator (due to the absence of allolactose), acting as a roadblock to RNA polymerase and preventing transcription.
- Glucose absent, lactose present: Strong transcription of the lac operon occurs. The lac repressor is released from the operator because the inducer (allolactose) is present. cAMP levels are high because glucose is absent, so CAP is active and bound to the DNA. CAP helps RNA polymerase bind to the promoter, permitting high levels of transcription.
References
- Principles of Genetics Sixth Edition by D. Peter Snustad and Michael J. Simmons; John Wiley & Sons, Inc.
- Genetics: A Conceptual Approach by Benjamin A. Pierce, 3rd edition 2009, WH Freeman and Company
- Biochemistry 4th edition 2010 by R. H. Garrett and C. M. Grisham, Brooks/Cole, Cengage Learning USA.
