Molecular Chapter 26

It codes for a protein that controls transcription by binding to a particular site on DNA

A trans-acting repressor protein binds to the cis-acting operator to turn off transcription.

A trans-acting protein binds to the cis-acting site in order for RNA pol to initiate transcription at the promoter

A unit of bacterial gene expression and regulation, including structural genes and control elements in DAN recognized by regulator gene products

A gene product that can diffuse to act on a target sequence

A DNA sequence that function only as a sequence and affects only the DNA to which it is physically linked

A gene that codes for a protein product

A genes ability to express due to the appearance of a protein product

A genes ability to be controlled by the amount of product made by the protein

A molecule that prevents the production of enzymes that are able to synthesize them

• - inducible
• - repressible
• + inducible
• + repressible

• 1. lacI - encodes the lac repressor (diffusable) and has its own promoter and terminator
• 2. Operator - lies between the promoter and lacZYA
• 3. lacZYA - encodes polycistronic mRNA for products that digest B-galactosides (e.g. lactose)

• 1. The operator overlaps the promoter
• 2. The repressor binds at the operator
• 3. Repressor binding prevents RNA pol from initiating ranscription at the promoter

• 1. Addition of the inducer results in rapid induction of lac mRNA
• 2. This is followed by enzyme synthesis
• 3. Removal of the inducer is followed by rapid cessation of synthesis

lac repressor is bound to the operator preventing RNA pol from initiating transcription

• 1. lac repressor is allosteric
• 2. The inducer binds the lac repressor and changes the DNA binding site on the repressor such that it has a lower affinity for the DNA lac operator
• 3. RNA pol binds the promoter and transcribes the operon

• 1. The operator can't bind the repressor
• 2. RNA pol has unrestricted access to the promoter
• 3. O^c are cis-acting because they affect only the contiguous set of structural genes

• 1. lacI^- mutations render the lac repressor inactive
• 2. Inactive lac repressor can't bind the operator
• 3. The operon is constitutively expressed

• 1. A lacI^-d mutant gene makes a monmer that has a damaged DNA binding site
• 2. When present in the same cell with the WT gene, multimeric repressors are assembled at random from both types of monomeric subunits
• 3. It only requires one of the subunits of the multimer to be of the lacI^-d type to prevent repressor from binding the operator DNA

• 1. Helix-Turn-Helix - DNA binding domain
• 2. Core - contains inducer binding site and multimerization binding sites

• 1. Consists of 2 dimers
• 2. Dimers are held together by interactions between the core subdomains 1 and 2

• 1. lacI^- - recessive and can't repress
• 2. lacI^s - dominant; either can't bind to DNA or can't be bound by inducer
• 3. lacI^-d - dominant negative; cna't bind DNA

The operator is palindromic

• 1. The DNA binding domain of lac repressor has a conformation that makes it easy to bind to successive major groves of the operator DNA
• 2. When the inducer binds lac repressor, it changes the orientation of the DNA binding domain and makes it harder for lac repressor to bind operator DNA

• 1. The DNA between the two binding sites forms a loop
• 2. This is important to full repression

• 1. The equilibrium constat for repressor/operator binding with inducer absent is 10⁷
• 2. The equilibrium constat for repressor/operator binding with inducer present is 10⁴
• 3. Repressor will bind 1000 times more easily when inducer is absent

• 1. Active repressor stays bound to operator
• 2. Repressor inactivated by inducer binds random low affinity DNA sites
• 3. When inducer leaves, repressor becomes active and is attracted by operator

• 1. cAMP activates inactive CRP
• 2. Active CRP binds the lac promoter and assists RNA pol in initiating transcription

• 1. cAMP is synthesized by adenylate cyclase
• 2. Glucose represses adenylate cyclase activity
• 3. No cAMP = inactive CRP = no transcription

Promoter-Operator-Leader-Attenuator-trpEDCBA-terminator-space-terminator'

• 1. It is controlled by the level of its product, tryptophan (autoregulation)
• 2. Tryptophan activates an inactive repressor encoded by trpR
• 3. The repressor will act on all 3 operator loci

The regulation of bacerial operons by controlling termination via changes in RNA secondary structure due to ribosome movement

• 1. An attenuator controls the progression of RNA pol into the trp genes
• 2. RNA pol initiates at the promoter and then proceeds before pausing at the attenuator
• 3. In the absence of Trp, RNA pol continues transcribing the trp structural genes
• 4. In the presence of Trp, RNA pol most likely terminates transcription

• 1. The expression of a short sequence that is located between the operator and the attenuator on the trp operon
• 2. The structure of mRNA at the attenuator depends on whether this reading frame is translated
• 3. In the presence of Trp-tRNA, the leader is translated to the leader peptide, and the attenuator is able to from the hairpin that causes termination

Regions 3 and 4 must be paired

• 1. In the absence of Trp, the ribosome stalls at the leader sequence
• 2. Regions 2:3 pair
• 3. RNA pol continues to the trp genes
• 4. In the presence of Trp, the ribosome translates the leader sequence and disrupts the 2:3 pairing
• 5. Regions 3:4 pair forming a hairpin
• 6. Termination

• 1. A regulator protein binds to the ribosome binding site
• 2. Ribosome can't bind to mRNA
• 3. No translation

The product of a first initiation site changes the mRNA secondary structure such that the second initiation site is blocked and translation stops

• 1. When rRNA is available, the r-proteins associate with it and translation of mRN continues
• 2. When no rRNA is available, r-proteins accumulate and begins to bind to mRNA and prevents translation