-
Two major modes of enzyme regulation
- – Activity
- • Temporary inactivation of the protein throughchanges in enzyme structure
- – Amount
- • Regulation at the gene level
-
Gene expression:
- -transcription of gene into mRNA followed by translation of mRNA into protein (Figure 8.1)
- - Most proteins are enzymes that carry out biochemical reactions
- - Constitutive proteins are needed at the same level all the time
- - Microbial genomes encode many proteins that are not needed all the time
- - Regulation helps conserve energy and resources
-
Two major levels of regulation in the cell
- – One controls the activity of preexisting enzymes
- • Posttranslational regulation
- • Very rapid process (seconds)
- – One controls the amount of an enzyme
- • Regulate level of transcription
- • Regulate translation
- • Slower process (minutes)
-
mRNA transcripts generally have a _____half-life (lenght)
- Short
- – Prevents the production of unneeded proteins
-
Regulation of transcription typically requires proteins that can bind to _____
DNA
-
______ molecules influence the binding of regulatory proteins to DNA
(size)
- Small
- – Proteins actually regulate transcription
-
Most DNA-binding proteins interact with DNA in a _____ manner
sequence-specific
-
DNA-Binding Proteins:
Specificity is provided by interactions between ___ ____ ____ ____ and chemical groups on the ___ ___ ___ ___ of DNA
- amino acid side chains
- bases and sugar–phosphate backbone
-
______ of DNA is the main site of protein binding
Major groove
-
___ ___ are frequently binding sites for regulatory proteins
Inverted repeats
-
Homodimeric proteins:
proteins composed of two identical polypeptides
-
Protein dimers
- interact with inverted repeats on DNA
- – Each of the polypeptides binds to one inverted repeat
-
Multiple outcomes after DNA binding are possible (2)
- 1. The binding event can block transcription(negative regulation)
- 2. The binding event can activate transcription(positive regulation)
-
Several mechanisms for controlling gene expression in bacteria (3)
- – environment in which the organism is growing
- – Presence or absence of specific smallmolecules
- – Interactions between small molecules and DNA bindingproteins result in control of transcription or translation
-
Negative control:
- -a regulatory mechanism that stops transcription
- -Repressor’s role is inhibitory, so it is called negative control
- ex: Repression, Induction
-
Repression:
- -preventing the synthesis of an enzyme in response to a signal (Figure 8.5)
- • Enzymes affected by repression make up a small fraction of total proteins
- • Typically affects anabolic enzymes (e.g., arginine biosynthesis)
- -used when nutrients are found in environment and arent synthesized
-
Induction:
- -production of an enzyme in response to a signal (Figure 8.6)
- • Typically affects catabolic enzymes (e.g., lac operon)
- • Enzymes are synthesized only when they are needed
- – no wasted energy
-
Inducer:
- substance that induces enzyme synthesis by removing repressor by binding to it.
- ex: lactose induces lactase synthase
-
Corepressor:
- substance that binds to repressor to represses enzyme synthesis.
- ex. argine binds to repressor to repress arginine synth
-
Effectors:
- -collective term for inducers and repressors
- -Effectors affect transcription indirectly by binding to specific DNA-binding proteins
-
Repressor molecules bind to an ___ ___ ___
allosteric repressor protein
-
operator
- -region of DNA near promoter that the Allosteric repressor becomes active and binds to
- – Operator is located downstream of the promoter
- – Transcription is physically blocked when repressor binds to operator (Figure 8.7)
-
Operon:
-cluster of genes arranged in a linear fashion whose expression is under control of a single operator
-
Enzyme induction can also be controlled by a ____
- repressor
- – Addition of inducer inactivates repressor and transcription can proceed (Figure 8.8)
-
Positive control:
- regulator protein activates the binding of RNA polymerase to DNA (Figure 8.9)
- ex: Maltose catabolism in E. coli
- – Maltose activator protein cannot bind to DNAunless it first binds maltose
- ? Activator proteins bind specifically to certain DNAsequence
- – Called activator-binding site, not operator
-
Activator proteins bind specifically to certain ____ sequence
– Called ___ ___ ___, not operator
- DNA
- activator-binding site
-
Promoters of positively controlled operons bind ___ly to RNA polymerase
(strength)
weakly
-
Activator protein helps RNA polymerase recognize ______
- promoter
- – May cause a change in DNA structure
- – May interact directly with RNA polymerase
-
Activator-binding site may be close to the promoter or ___ ___ base pairs away(Figure 8.11)
- several hundred
- -can effect the bonding by bending DNA
-
How are genes for maltose situated on the chromosome?
Genes for maltose are spread out over the chromosome in several operons
-
Each operon has a(n) ___-___ ___
activator-binding site
-
regulon
- -Multiple operons controlled by the same regulatory protein
- -Regulons exist for positive and negatively controlled systems
-
Global control systems:
- regulate expression of many different genes simultaneously
- different term for regulon
-
Catabolite repression
- -an example of globalcontrol
- – Synthesis of unrelated catabolic enzymes is repressed if glucose is present in growth medium(Figure 8.12)
- – lac operon is under control of catabolite repression
- – Ensures the “best” carbon and energy source isused first
-
Diauxic growth:
- -two exponential cell growth phases
- -ex: glucose used up completely before lactose is used
-
– Cyclic AMP receptor protein (CRP)
- -an activator protein that controls transcription in catabolite repression
- -is a form of positive control
-
Cyclic AMP
- -a key molecule in many metabolic control systems
- -highly produced with lack of glucose
- • It is derived from a nucleic acid precursor
- • It is a regulatory nucleotide
-
Effects catabolite repression
- - Dozens of catabolic operons affected by catabolite repression
- – Enzymes for degrading lactose, maltose, and other common carbon sources
- - Flagellar genes are also controlled by catabolite repression
- – No need to swim in search of nutrients
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