Molecular Genetics
Operator Regulation--Genetics
Facts | Interpretations | Further Info. | Other Pages
Traditional genetics, the isolation of mutations conferring particular phenotypes followed by their characterization by genetic and molecular means, is one effective way of deciphering signals involved in transcription.
Facts
|
Uninduced |
Induced |
- Jacques Monod and coworkers (ref) identified several genes whose mutation altered the inducible synthesis of b-galactosidase by E. coli.
- Some mutations in a gene they called operator, oC, resulted in b-galactosidase synthesis without induction (o constituitive). Others, oo mutations, completely prevented enzyme synthesis.
- Some mutations in another gene, the igene, also resulted in constitutive expression (i- mutations).
|
| o+ z+ |
< 0.1 |
100 |
| ocz+ |
15 |
90 |
| o+ z+/oc z- |
< 0.1 |
90 |
| o+ z-/oc z+ |
90 |
250 |
| i+ oo z+ |
< 0.1 |
< 0.1 |
| i- o+ z+ |
200 |
200 |
| i+ oo z+/i- o+ z+ |
1 |
260 |
- The two constituitive mutations i- and oC mapped to different loci and behaved differently in partly diploid bacteria. The i+ allele dominated over i- and acted in trans. The oC mutations were not dominant and did not act in trans.
- Based on these and other results, Monod proposed his now famous operon model.
|
Facts | Interpretations | Further Info. | Other Pages
Interpretations
- Monod's approach is an example of the genetic approach to understanding the regulation of transcription.
- Beta-galactosidase synthesis is an example of induction. A small molecule, a co-inducer, induces enzyme synthesis.
- b-galactosidase regulation is also an example of negative regulation, where binding of the protein inhibits transcription.
Facts | Interpretations | Further Info. | Other Pages
Further information
- Repression systems, in which the presence of a small molecule, a co-repressor, turns gene expression off, also exist.
- Inhibition is in part due to occlusion of the promoter. In some other systems, examples of positive regulation, protein binding stimulates transcription. Positive regulation of b-galactosidase synthesis also occurs: synthesis is stimulated by binding of the cyclic AMP binding protein. Positive regulation is thought to act by altering DNA conformation.
- In some cases, the inducing (or repressing signal) is extracellular. In many such cases, the regulators consist of two components: a membrane sensor and a DNA-binding protein. The stimulated sensor activates the DNA-binding protein.
- The expression complex controlled by a single operator is called an operon. Multiple operons controlled by the same repressor or activator are called a regulon.
- Quiz yourself on lac operon genetics.
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This is page 2224 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2001
E-mail inquiries to U. Melcher------------Last Revision: 3 November, 2003