Lecture 12 - Chapter 11 pt. 1 PDF

MICR 321: Advanced Microbiology Lecture 12: Chapter 11: Regulation of Gene Expression: Genes & Operons Regulation of Gene Expression Cells express only the genes they need in a particular environment. Why? Expression of genes can occur at different levels Tra...

MICR 321: Advanced Microbiology Lecture 12: Chapter 11: Regulation of Gene Expression: Genes & Operons Regulation of Gene Expression Cells express only the genes they need in a particular environment. Why? Expression of genes can occur at different levels Transcriptional regulation Posttranscriptional regulation Translational regulation Posttranslational regulation Most transcriptional regulation occurs at the promoter through proteins called transcriptional regulators Repressors - binds to an operator sequence in the DNA and prevents initiation of transcription, ie. negative regulation Activators – required for initiation of transcription by RNA polymerase  Khan Academy Regulation of Gene Expression Transcriptional Regulation Regulators can be repressors, activators, or both Regulation can be negative, positive, or both Inducers – increases gene expression activate an activator OR inactivate a repressor Co-repressor – decreases gene expression activate a repressor OR inactivate an activator Repressible system – biosynthetic pathways that are regulated by the end- product, ie. only on when necessary Constitutive mutant – a mutant in which the genes of an operon are always transcribed even in the absence of inducer → leads to constitutive expression How do we find these things in the genome??  Box 11.1 Helix-Turn-Helix motifs Transcriptional Regulation Activators Repressors Activator can bind and recruit Simplest mechanism: steric RNA polymerase hindrance Activator can stabilize RNA Repressor directly inhibits RNA pol from binding or polymerase binding to promoter proceeding Not all promoters are great! Repressors can also change DNA Can promote open-complex structure formation Inhibit RNA pol binding or Promotes frequency of proceeding initiation Can also work as an anti-activator Can also function to inhibit a Affects positive activity of an repressor activator Negative regulation of operons Catabolic operons - degrade compounds to obtain catabolites to build other molecules Biosynthetic operons - synthesize compounds needed by the cell such as amino acids, nucleotides, and vitamins Biosynthetic operons should not be turned on when the end product of the pathway is available Negative regulation - genes in an operon are constitutively expressed unless they are turned off by a repressor protein Co-repressors bind to the repressor and allows it to bind to the operator site, thus decreasing gene expression. A repressor that negatively regulates a biosynthetic operon will be inactive without the co-repressor. This state is termed aporepressor.  Figure 11.14 E. coli ara operon – Positive regulation Positive regulation of operons - under control of an activator protein & only transcribed when activator is bound Classic example of positive inducible system BUT can also function as an antiactivator Encodes enzymes involved in the utilization of arabinose 3 structural genes (araABD), promoter (PBAD) Divergently encoded araC – transcriptional activator, promoter (PC)  Figure 11.15 E. coli ara operon AraC has two functions and exists in two protein states P1 & P2 Absence of arabinose (anti-activator): AraC P1 binds to araO2 and araI1 → cannot activate transcription Presence of arabinose (activator): AraC P2 binds to araI1 and araI2 → activates the operon Autoregulation of AraC: High concentrations AraC binds operator araO1 → Preventing transcription from Pc