Gene Regulation: Positive and Negative Control Mechanisms

Gene Regulation Mechanisms

• Regulator genes encode products that regulate the expression of other genes.
• Positive control mechanisms require the product of the regulator gene (activator) to turn on the expression of structural genes.
• Negative control mechanisms require the product of the regulator gene (repressor) to shut off the expression of structural genes.

Negative Regulation

• Molecular signal causes binding of regulatory protein to DNA.
• Bound repressor inhibits transcription.

Positive Regulation

• Molecular signal causes dissociation of regulatory protein from DNA.
• Bound activator facilitates transcription.

Bacterial Operon

• Consists of activator binding site, repressor binding site (operator), promoter, and regulatory sequences.
• Genes transcribed as a unit (polycistronic mRNA) and translated into multiple proteins.

Gene Regulation and Expression

• Nucleus from an adult skin cell can be injected into an enucleated egg to produce a normal embryo and tadpole.
• Many stages of gene expression can be regulated, and the rate of each can be changed.

Constitutive Genes

• Essential for almost all living cells.
• Continuously expressed in most cells.
• Perform "housekeeping" functions.
• Examples include genes involved in protein synthesis, such as ribosomal RNA, transfer RNA, and RNA polymerase subunits.

Inducible and Repressible Genes

• Only needed under certain environmental conditions.
• Synthesis controlled by regulatory mechanisms.
• Examples include alcohol dehydrogenase, which is only made when ethanol is present.

E. coli RNA Polymerase

• Core enzyme composed of α, β, β', and ω subunits.
• Holoenzyme has an additional σ subunit.
• Subunit functions:
  • α: Assembly of the tetrameric core.
  • β: Ribonucleoside triphosphate binding site.
  • β': DNA template binding region.
  • σ: Initiation of transcription.

Regulation at Transcription Initiation

• Three main ways to regulate transcription initiation:
  • Alter specificity of RNA polymerase-promoter interaction.
  • Impede access of RNA polymerase to its promoter.
  • Use of activators to enhance polymerase-promoter activity.

DNA Sequences Involved in Regulation of Gene Expression

• Bacterial promoters include conserved -10 region (Pribnow box) and -35 region.
• These regions interact with the σ factor of RNA polymerase.
• Some promoters also include the upstream element, which interacts with the α subunit of RNA polymerase.

Two Main Mechanisms to Regulate Transcription in Bacteria

• Use of different σ factors in RNA polymerase.
• Binding of other proteins, transcription factors, to promoters.