regulation of gene and cellular activity. cells use different mechanisms to sense and respond to...
Post on 21-Dec-2015
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• Cells use different mechanisms to sense and respond to conditions within or outside the cell.
• Regulatory proteins help a cell sense internal changes and alter its gene expression to match.
Regulating Gene Expression
Figure 10.1A
• Microbes control gene expression at several levels:
• - Alterations of DNA sequence• - Control of transcription • - Control of mRNA stability• - Translational control• - Posttranslational control
Regulating Gene Expression
• The lactose utilization lacZYA operon of E. coli was the first gene regulatory system described.
• First, here’s how lactose is transported and metabolized.
The E. coli Lactose Operon
Figure 10.6
• LacI binds as a tetramer to the operator region.• - It represses the lac operon by preventing open
complex formation by RNA polymerase.
Scenario 1: Absence of Lactose
Figure 10.5B
• b-galactosidase (LacZ), when at low concentrations, cleaves and rearranges lactose to make the inducer allolactose.
• Allolactose binds to LacI, reducing its affinity to the operator and thus allowing induction of the operon.
Scenario 2: Presence of Lactose
Figure 10.5C
• Maximum expression of the lac operon requires the presence of cAMP and cAMP receptor protein (CRP).
• - The cAMP-CRP complex binds to the promoter.• - Interacts with RNA pol to increase the rate of
transcription initiation
Activation of the lac Operon by cAMP-CRP
Figure 10.8
Catabolite Repression• In catabolite repression, an
operon enabling the catabolism of one nutrient is repressed by the presence of a more favorable nutrient (commonly glucose).
The biphasic curve of a culture growing on two carbon sources is often called diauxic growth.
Figure 10.10
Catabolite Repression
• Glucose transport by the phosphotransferase system causes catabolite repression by inhibiting the LacY permease activity.
• - This is termed inducer exclusion.
Figure 10.11
• Attenuation is a regulatory mechanism in which translation of a leader peptide affects transcription of a downstream structural gene.
Attenuation of the trp Operon
Figure 10.16A
The attenuator region of the trp operon has 2 trp codons and is capable of forming stem-loop structures.
• Quorum sensing refers to the process where bacterial cells work together at high density.
Quorum Sensing
- It was discovered in Vibrio fischeri, a bioluminescent bacterium that colonizes the light organ of the Hawaiian squid.
Figure 10.30ACD
• Animation: Chemotaxis: Molecular Events
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Chemotaxis: Molecular Events