Microbiology Final Exam Review - December 2023 PDF

Microbiology Final Exam Review Tuesday, December 10 8AM TR Ch 10, Molecular Regulation Overview Introduction to gene regulation Transcription repressors and activators The lactose operon Introduction A bacterial genome encodes thousands of different proteins. Cells do not ex...

Microbiology Final Exam Review Tuesday, December 10 8AM TR Ch 10, Molecular Regulation Overview Introduction to gene regulation Transcription repressors and activators The lactose operon Introduction A bacterial genome encodes thousands of different proteins. Cells do not express every gene at maximal levels due to: ○ Physical space limitations ○ Energy and resource conservation ○ Contradictory functions Mechanisms of Gene Regulation Microbes use numerous mechanisms to sense their internal and external environments, which direct the synthesis of specific proteins. Gene regulation occurs at multiple levels: ○ Alteration of DNA sequence ○ Control of transcription ○ Control of mRNA stability ○ Translational control ○ Post-translational control Transcription Repressors and Activators Transcription Initiation: Major site of regulatory control in bacteria, often controlled by regulatory proteins. Regulatory Proteins: ○ Bind DNA at or near gene promoters ○ Stimulate or prevent binding of RNA polymerase to promoter ○ Interact with DNA's major groove, often forming dimers DNA Targets: ○ Exhibit symmetry in the form of inverted repeats ○ Regulatory proteins often bind as dimers, each monomer binding to one of the inverted repeat sequences. Mechanisms: ○ Intracellular changes detected by regulatory proteins binding specific low-molecular-weight compounds called ligands. ○ Genes encoding regulatory proteins are transcribed separately from the target gene. 14  Microbiology Final Exam Review Tuesday, December 10 8AM TR Forms of Regulatory Proteins Repressors: ○ Prevent gene expression by binding to DNA sequences called operators. ○ Some repressors bind in the absence of a ligand (inducer); others bind only after binding a ligand (corepressor). Activators: ○ Stimulate gene expression by contacting RNA polymerase positioned at a nearby promoter, encouraging it to initiate transcription. ○ Bind poorly to DNA sequences unless bound to their ligand (inducer). Sensing the Extracellular Environment Two-Component Signal Transduction Systems: ○ Sensor Kinase: Membrane-bound enzyme that binds environmental signals and transfers a phosphate group from ATP to another target protein. Sensory domain contacts the outside environment, while the kinase domain protrudes into the cytoplasm. ○ Response Regulator: Cytosolic protein stimulated by the activated sensor kinase to bind DNA and either stimulate or repress gene expression. Controlled by covalent modifications, down-regulated when a phosphatase cleaves the phosphate from the response regulator. The Lactose Operon Historical Context: ○ In 1961, Jacques Monod and François Jacob proposed gene regulation, noting that E. coli lactose metabolizing enzymes were inducible, while glucose metabolizing enzymes were constitutive. ○ They shared the 1965 Nobel Prize in Physiology or Medicine with André Lwoff. Lactose Metabolism: ○ Disaccharide made of glucose and galactose, used as a carbon and energy source. ○ Requires: Lactose Permease (LacY): Transports lactose into the cell. β-Galactosidase (LacZ): Cleaves lactose into monosaccharides or rearranges lactose into allolactose. Regulation of the lac Operon Operon Structure: ○ Genes lacZ, lacY, and lacA form an operon, regulated by a single promoter (PlacZYA) and operator sequences (lacO). ○ The lactose repressor (LacI) is encoded by a regulatory gene situated upstream of the operon and expressed constitutively from its own promoter. Lactose Presence: 15  Microbiology Final Exam Review Tuesday, December 10 8AM TR ○ In the absence of lactose, LacI binds the operators lacO and lacOI, preventing RNA polymerase from initiating transcription. ○ In the presence of lactose, allolactose binds LacI, reducing its affinity for the operator, allowing RNA polymerase to initiate transcription of the lacZYA genes. cAMP and CRP Mechanism: ○ cAMP accumulates when a cell is starved for carbon and combines with cAMP receptor protein (CRP). ○ Maximum expression of the lactose operon requires the removal of the lactose repressor (LacI) and the binding of the cAMP-CRP complex. ○ The cAMP-CRP complex binds to a DNA sequence upstream from the start of transcription, bending the DNA and interacting with RNA polymerase to activate transcription. Catabolite Repression Mechanism: ○ Catabolite repression occurs when an operon enabling the catabolism of one nutrient is repressed by the presence of a more favorable nutrient (e.g., glucose). ○ Diauxic Growth: Biphasic curve of a culture growing on two carbon sources. ○ Inducer Exclusion: Glucose transport by the phosphotransferase system (PTS) inhibits LacY permease, preventing lactose from entering the cell and inducing the lac operon. ○ In the absence of glucose, phosphorylated forms of glucose-specific IIAGlc and IIBGlc accumulate, allowing LacY to transport lactose and induce the lac operon. 16