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Biol 240 Exam #1 Review Notes

4 Pages

Course Code
BIOL 240
Josh Neufeld

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Riboswitches  Resemble repressors and activators  Region upstream of the coding sequence (5’ end); Non-coding region  Fold into specific 3D structure that binds small molecules  Binding of a metabolite alters the structure of the riboswitch domain, preventing translation o Structure blocks Shine-Dalgarno site and prevents ribosomes from binding to it  High concentration of thiamine pyrophosphate = metabolite binds, translation blocked  Low concentration of thiamine pyrophosphate = metabolite dissociates, translation  Controls translation, but may also control transcription Attenuation  Controls transcription  Premature termination of mRNA synthesis (after initiation but before termination)  Leader region (after promoter and operator) folds up into 2 alternative secondary structures o 1 structure allows continued RNA synthesis o Other structure causes premature termination  Best example: regulation of gene controlling the biosynthesis of certain amino acids in gram- negative bacteria  Attenuation is absent from Eukarya b/c transcription and translation and spatially separated!  Leader sequence encodes a short polypeptide (leader peptide) that functions as an attenuator o If trp is plentiful, tryptophan tRNAs and the leader peptide will be synthesized o If trp is low, the tryptophan-rich leader peptide will not be synthesized o Synthesis of leader peptide results in termination of transcription o If synthesis of the leader peptide is blocked (low trp level),the operon is transcribed  mRNA folds into a stem-loop that terminates RNA polymerase activity o 2 stretches of nucleotides near each other are complementary and can base-pair o If trp is plentiful, ribosomes translate leader sequence until the stop codon; remainder forms a stem-loop followed by a uracil-rich sequence (intrinsic terminator); termination o If trp is low, ribosomes pause at trp codon of leader sequence, which allows a stem-loop to form (sites 2 and 3) and prevents terminator stem-loop (sites 3 and 4) from forming  Leader peptide for each amino acid biosynthetic operon is rich in that particular amino acid! Quorum Sensing  Regulation of gene expression based on population density (of the same species) o Bacteria asses their population density o Ex. a pathogenic bacterium that secretes a toxin can have no effect as a single cell (would be a waste of energy); however, if there were more cells present, the coordinated expression of the toxin may successfully initiate disease  Widespread among gram-negative bacteria, although also found in gram-positive  Each species that employs quorum sensing synthesizes a signal molecule called an autoinducer o Fuses freely across the cell envelope in either direction o Autoinducer binds to an activator protein and triggers transcription of specific genes  Positive control of transcription  Class of autoinducer – acyl homoserine lactones (AHLs) Two-Component Regulatory Systems  Signal transduction – external signal is detected by a sensor that transmits it to the rest of the regulatory machinery o contain 2 parts; therefore called two-component regulatory systems o consist of 2 different proteins: sensor kinase protein (in cytoplasmic membrane) and a partner response regulator protein (in the cytoplasm)  kinase – phosphorylates compounds (using phosphate from ATP)  sensory kinase – detects a signal from the environment and phosphorylates themselves (autophosphorylation) at a specific histidine residue on the cytoplasmic surface o sensory kinase = histidine kinase  phosphoryl group is transferred to another protein inside the cell called the response regulator o DNA
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