BBMB 405 1st Edition Lecture 35 Outline of Last Lecture XVI Chapter 30 Protein Synthesis C Eukaryotic protein synthesis differs from prokaryotic protein synthesis primarily in translation initiation D A variety of antibiotics and toxins can inhibit protein synthesis E Ribosomes bound to endoplasmic reticulum manufacture secretory and membrane proteins XVII Chapter 31 The control of gene expression in prokaryotes A Introduction Outline of Current Lecture XVII Chapter 31 The control of gene expression in prokaryotes A Introduction B Many DNA binding proteins recognize specific DNA sequences C Prokaryotic DNA binding proteins bind specifically to regulatory sites in operons Current Lecture XVII Chapter 31 The control of gene expression in prokaryotes B Many DNA binding proteins recognize specific DNA sequences C Prokaryotic DNA binding proteins bind specifically to regulatory sites in operons 1 The lac operon classic example of transcriptional regulation These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute a b Bacteria need to control gene expression because their transcription and translation are combined c Activators or inhibitors bind to control sight and turn genes off or on 2 Observation Lac operon production increases as presence of lactose increases a Lactose metabolism proteins are only expressed in the presence of lactose b There has to be a mechanism for sensing lactose and turning on gene when regulatory gene and operon are removed the lactose structural genes are expressed all the time 3 Reminder sugar nomenclature Review chapter 11 Beta galatosides have an organic R group on the C1 oxygen lactose galactosyl glucose is an example 4 The lac operon controls lactose transport and metabolism in E coli a Lac Y lactose permease b c Lac Z beta galactosidase d e Beta galactosidase can also be used to cleave X gal which produces a blue color this is used to assay activity of beta galactosidase f Lac A thiogalactoside transacetylase Acetyl CoA Beta D galactoside CoA 6 acetyl Beta D galactoside Not essential for lactose metabolism May have role in detoxification of compounds transported by permease g Lac I the lac repressor is an inverted repeat The lac repressor binds the lac operator Binds to the major groove using Arg Operator DNA is a symmetrical dimer of lac repressor 5 Repressors block transcription of genes a b Lac repressor DNA binding Dissociation constant Kd of 0 1 pM 10 13 for operator sequence Diffuses along DNA to search for operator 1D not 3D search c Repressor inducer complex does not bind DNA Inducer binding causes conformational change in repressor Several Beta galactosides are inducers of Lac operon 6 No inducer present A figure above Lac repressor binds the operator tightly 7 Inducer present B figure above lac repressor releases operator note DNA binding domains are disordered illustration is organized for show 8 Lac repressor binds two sites as a tetramer a Two sites within 500 bp of primary operator have similar sequence b One dimer binds operator the other binds at one of the other two sites c Intervening sequence loops out d No other closely matching sites in E coli genome 9 Lactose is not direct inducer of the lac operon The lac operon is induced in the presence of 1 6 Allolactose 10 The lac operon paradox a Beta galactosidase can catalyze two reactions using lactose as substrate either hydrolysis to create glaactose and glucose or transgalactosylsis to create 1 6Allolactose b A small amount of beta galactosidase is necessary to create an inducer from lactose 11 When would E coli need to metabolize lactose a What is preferred energy source Glucose b When glucose is abundant other carbon sources are not necessary high glucose concentrations inhibit lactose permease and adenylate cyclase c When glucose is depleted lactose metabolism or other carbon source will be upregulated by catabolite activator protein CAP 12 Helix turn helix DNA binding motifs a Second helix of N helix turn helix C contacts major groove of DNA b Amino acid side chains make contact with edges of base pairs 13 Multiple operons can be controlled by same or similar operator sequences a Pur repressor binds DNA when bound to guanine hypoxanthine co repressors rather than inducers negative feedback regulator b Pur operator sequence is short 5 ANGCAANCGNTTNCNT 3 evolved independently at sites around genome regulatory regions of genes participating in nucleotide biosynthesis
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