20 - Transcription (Prokaryotes); DNA Binding Proteins & Initiation

20 - Transcription (Prokaryotes); DNA Binding Proteins & Initiation

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Basic review: Lactose operon: •Catabolic •Negative regulation •Regulatory protein is repressor ; effector = inducer: allolactose (converted lactose) •Existence of catabolite activator protein (CAP) Tryptophane operon: •Anabolic •Negative regulation •Regulatory protein is aporepressor ; effector = co-repressor: tryptophan •Autoregulation L-arabinose operon •Catabolic •Positive and negative regulation • Same regulatory protein acts as both activator and repressor (=regulator) – two different conformations / corresponding binding sites effector = inducer: arabinose •Autoregulation of AraC and CAP regulation of the operon
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Operator regions – more complicated then previously shown Lac -operon region contains 3 operators: •Strongest – commonly shown O1 •Downstream O2 – inside lacZ reading frame •Upstream O3 All three present – transcription suppressed 1000-fold •If either O2 or O3 are missing – 500-fold •Both O2 and O3 missing – 20-fold Regulation of transcription in Prokaryotes (cont.) – Multiple operators – auxiliary operators Also: presence of auxiliary operators (O2 and O3) near the functional operator (O1) increases the local concentration of the repressor , so that it can occupy the functional operator (O1) Repressor can bind as a tetramer – could bind to all three operators (2 at time) – causes DNA looping – RNAP prevented from binding to promoter
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(a) Schematic of the regulatory elements located in lac operon DNA. P denotes the promoter, O1, O2 and O3 correspond to the three operators (repressor-binding sites), and C is the binding site for the cAMP–CRP complex. Santillán M , Mackey M C J. R. Soc. Interface 2008;5:S29-S39 ©2008 by The Royal Society (a) Schematic of the regulatory elements located in lac operon DNA. P denotes the promoter, O1, O2 and O3 correspond to the three operators (repressor-binding sites), and C is the binding site for the cAMP–CRP complex. The different ways in which a repressor molecule can interact with the operator sites are represented in b, c, d and e. Namely, a free repressor molecule (b), one with a single subunit bound by allolactose (d) or one with the two subunits in the same side bound by allolactose (e) can bind a single operator. Moreover, a free repressor molecule can bind two different operators simultaneously (c).
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Two component regulatory systems Regulation of transcription in Prokaryotes (cont.) – 1. One protein is the sensor-transmitter protein - monitors specific changes in the environment (level of nutrients, pH, solvent concentration-osmolarity etc.); kinase 2. Second protein is response regulator protein - either stimulates or represses regulation of specific genes – changes in gene expression necessary for bacterium to adapt to environmental change
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Two component regulatory system (cont.) Sensor-transmitter
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20 - Transcription (Prokaryotes); DNA Binding Proteins & Initiation

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