I. Introduction
- Proteins are often used to regulate transcription initiation, and therefore induction and repression.
II. Transcriptional Regulatory Proteins
- Many of these transcriptional regulatory proteins are DNA-binding proteins that form dimers and attach to short, inverted sequences of bases in the DNA called palindromes.
- Only a small portion of the proteins interacts with the palindromes, and these are referred to as DNA binding domains.
- Examination of numerous transcriptional regulatory proteins has led to the recognition of two common motifs in DNA-binding domains: helix-turn-helix and zinc fingers.
- Helix-turn-helix DNA-binding domains are often observed in transcriptional regulatory proteins.
- Zinc finger DNA-binding domains have been observed in some bacterial regulatory proteins.
III. Negative and Positive Transcriptional Control
- Regulatory proteins can exert either negative or positive control.
- Negative transcriptional control occurs when the binding of the protein to DNA inhibits the initiation of transcription. Regulatory proteins that act in this fashion are called repressor proteins.
- Positive transcriptional control occurs when the binding of the protein to DNA promotes transcription initiation. These proteins are called activator proteins.
IV. Repressor and Activator Proteins
- Repressor and activator proteins usually function by binding DNA at specific sites.
- In bacteria, repressor proteins attach to a region called the operator, which usually overlaps or is downstream of the promoter (i.e., closer to the coding region).
- Activator proteins attach to activator-binding sites. These are often upstream of the promoter (i.e., farther away from the coding region).
V. Regulation of Repressor and Activator Proteins
- Repressor and activator proteins must exist in both active and inactive forms of transcription initiation to be controlled appropriately.
- Their activity is usually modified by small effector molecules, most of which bind the repressor or activator protein noncovalently (i.e., allosteric regulation).
- Figure 14.3 shows the four basic ways in which the interactions of an effector and a regulatory protein can affect transcription.
VI. General Aspects of Regulation
- Gene expression is rarely an all-or-nothing phenomenon; it is a continuum.
- The second aspect of regulation is the “decision-making” process used by microbial cells.
- Consider the regulatory “decisions” made by an E. coli cell.
VII. Conclusion
- The regulation of transcription initiation is a complex process that involves many different types of regulatory proteins and mechanisms.
