Flashcards in Lecture 10 Deck (24):
Regulating translation initiation:
- Protein binds mRNA regulating initiation at SD/AUG (influencing ribosomal binding)
- sRNA binds mRNA regulating initiation at SD/AUG
- Structure in mRNA inhibits initiation at SD/AUG, could be a small MW effector molecule or a ribosome translating an upstream ORF.
- Most ORF have their own SD sequence with the initiation codon at the start
- Each ORF is translated by independent ribosomes
- Complementary to the 3' end of the 16s RNA in the ribosome, so it positions the ribosome ready for translation
General mechanisms for regulation translation initiation:
- AN RNA-binding protein prevents translation initiation by blocking the ribosome site
- A secondary structure in the mRNA prevents translation initiation of the second ORF, so translation of the first ORF disrupts the structure allowing translation.
- Large subunit
- Small subunit
- Expressed in a series of different operons
Regulation of ribosomal proteins:
- Some ribosomal proteins interact with rRNA forming an integral structure.
- If rRNA is in excess, proteins will be used.
- If ribosomal proteins are in excess, protein 2 binds to mRNA preventing translation of protein 1.
- Translation of ORF2 is inhibited by structure in mRNA
eg) Ribosomal S8 normally binds structure A in the 16S rRNA:
- If in excess, ribosomal protein S8 binds structure B in the mRNA encoding S9 thus inhibiting translation of itself
- Feedback regulation
- can regulate translation initiaition and influence transcription termination
eg) SAM is a small MW effector molecule that alters the structure of mRNA to block the ribosome binding site, blocking transcription.
Trans-acting small RNAs (sRNAs):
- Can regulate translation initiation positively or negatively
- sRNA binds to mRNA inhibiting formation of secondary structures, allowing ribosome to initiate at SD/AUG
- or Preventing translation initiation at SD/AUG
Osmolarity response - involve an sRNA
- The regulator allows one gene to be positively regulated while the other gene is effectively negatively regulated.
- small RNA response
a protein that forms pores in the membrane
A protein that forms pores in the membrane
- A porin gene
- positively regulates ompC
- positively regulates the small RNA gene, micF
- Inhibits ompF expression
- A negative regulator of ompF
- A small RNA gene, which binds to ompF mRNA inhibitin translation
Regulation of translation termination eg) Translation release factor 2 (RF2) regulates its own expression:
- Excess RF2: ribosomes terminate RF2 translation at codon 26, and expression doesn't occur
- Limiting RF2: ribosomes do not terminate RF2 translation, ribosomes shift reading frame, due to an internal SD sequence that repositions the ribosome by 1 base, to translate full ORF
How does density effect bacterial gene expression?
- Cells can uniformly change in some way by activating expression of response genes
- A signal produced reaches a critical threshold which activates a synchronised response
Examples of this:
- Disease causing bacteria expressing disease causing genes only once they reach a certain level of population density
1. The trp operon is regulated by an attenuation mechanism
2. Some trpR- mutants are dominant (ie: trpR-D)
3. Tryptophan is the effector of the trpR regulatory protein
4. Transcription termination involves the formation of a stem-and-loop in the RNA
The trp operator is a region of dyad symmetry. Explain: 2 gives the best reasoning
2. TrpR is a dimer, so a mutant monomer can interfere with the function of a WT monomer and these are generally mutations in the DNA binding domain.
4. Region of dyad symmetry, but nothing todo with the operator or the binding of TrpR
Unable to undergo lysis or form a lysogen (lethal mutation)
N mutant: because N is needed to progress the intermediate early genes, and if this is mutated it is lethal
Unable to establish a lysogen
cI mutant: because lambda repressor is inactive and PR and PL cannot be repressed
Unable to undergo lysis
Q mutant: unable to expressed late genes
Xis: could undergo lysis, could undergo lysogeny, could integrate, but wouldn't be able to excise, so it can't fully enter the lytic cycle
Unable to both undergo lysis OR form a lysogen but lysogen cannot be induced to undergo lytic cycle
nutL mutant: required for cIII expression, more likely to undergo lysis, possibly unable to establish a lysogen