13BIS101S2013GeneRegulLect13-1

G glucose lactose maltose arabinose etc consistent

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Unformatted text preview: ene Expression R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 26 Operon Model: CAP and Positive Regulation CAP-cyclic AMP Binding to CAP site BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 27 Lactose Operon: Positive Control Catabolite Repression and Positive Regulation: Unlike the controlled environment of the laboratory, bacteria in nature frequently find themselves in mixed chemical environments containing a mixture of sugars to use as a carbon source (e.g., glucose, lactose, maltose, arabinose etc). Consistent with the Law of Parsimony, cells will utilize the Consistent simplest carbon source (e.g., glucose) before they use more complex sugars. To utilize the more complex sugars before all glucose is expended would require the wasteful synthesis of the enzymes to break down the more complex sugars. This would be a needless use of the cell's energy and resources (e.g., transcriptional and translational machinery). resources How does the cell know to use glucose first and ignore How the more complex sugars? the BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 28 CAP Binding to the Lac Promoter In the presence of In glucose and in the absence of lactose, repressor binds the operator. operator. In presence of glucose In and lactose, a small and amount of lac mRNA is made because there is no cAMP/CAP complex. no In the absence of In glucose and presence of lactose, lac mRNA transcription is high due the help of cAMP/CAP. the BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression R.L. Rodriguez 29 Catabolite Repression and Positive Control The wild type lac promoter is weak because of a nonconsensus -10 The lac region (TATGTTG) instead of TATPuATG. (The mutant lac UV5 promoter lac is TATAATG). Therefore, even when the repressor is bound by allolactose and prevented from binding to the lac operator, RNAP holoenzyme binds lac the promoter but is slow to form the open complex (also called isomerization). Isomerization needs help from a positive activator isomerization). protein called CAP (catabolite activator protein) to stimulate open CAP complex formation and subsequent transcription initiation. complex CAP must be first complex with cyclic AMP (cAMP) before it can bind , CAP cyclic (as a dimer) to the palindromic sequence upstream of the -35 region of the lac promoter. CAP/cAMP binds these sequences and bends the DNA lac CAP/cAMP 90°. This enhances isomerization at -10 by RNAP holoenzyme and transcriptional activity. transcriptional The concentration of glucose in the cell is inversely proportional to The cAMP concentration (i.e., the higher the glucose concentration, the lower the concentration of cAMP). When glucose is low, an unknown catabolite of glucose is low. This allows the accumulation of the phosphotransferase, EIIA, which in turn, increases the level of activated adenyl cyclase (responsible fo...
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