Lecture 14 Stop Transcription Flashcards
Antitermination
Allows terminator signal to be ignored, antitermination attached to RNA polymerase at promotor region allowing it to skip one terminator
Important in infection strategy of phage Lambda
Attenuation in Trp operon
A type of antitermination using ribosomes. Another regulating mechanism for Tryp operon.
Leader region - region of transcribed DNA.
Usually transcripts go from leader all the way through to last gene but not many of them.
Mutants with a deletion at the end of leader region produce a large no. Of transcripts
If you delete something and process becomes stronger you’ve deleted a neg regulator e.g. a terminator
In wild type bacteria most transcripts are short and only reach end of promotor region - regulated by presence of tryptophan. So terminator sequence at end of leader.
TrpL region (leader region)
Potential for 3 stem loops to form but can’t form all at once.
If first and last pair (the terminator) form then the central loop cannot form.
If loop 2 forms then loop 1 and 3 cannot form so terminator not formed and transcription continues
- RNA polymerase skips the terminator as it is not expressed
Translation in bacteria
Translation of mRNA starts as soon as RNA polymerase has made it - possible as bacteria has no nuclear envelope
High quantities of Trp available
When high quantities of Trp available
Ribosome incorporates 2 tryptophan. 1st stem loop transcribed. Peptide translated and ribosome disocciates. 1st stem loop forms allowing 3rd stem loop to form (terminator) so no more Tryp synthesised
Short gene - open reading frame (ORF)
ORF= DNA that can be translated and transcribed to a protein
Low Trp available
Ribosome attempts to insert 2 tryptophan to continue, none available so it stalls, gets stuck and cannot dissociate preventing loop 1 from folding. RNA polymerase transcribes stem loop 2 and second loop forms preventing terminator from forming so that more tryptophan is treanscribed
Summary
Stem loops in leader mRNA determine whether Tryp genes E-A are transcribed
Not needed - short transcript ends after leader
Needed - full transcript
Attenuation in other operons
E.g. phenylalanine, ribosome stalling point is 7 adjacent Phe residues
Attenuation summary
Terminator in mRNA upstream of operon genes
Terminator formation prevented if ribosome stalls at crucial position
Ribosome is translating a short ORF into a protein that requires the amino acid in question
So if amino acid is present ribosome won’t stall and terminator will be formed
Bacteriophage
Lifecycle governed by coordination of certain genes switched on when needed chooses what to do “ makes a choice”
Bacteriophages infect bacteria
There are 3 types
Icosahedral - hexagonal shape
Filamentous - linear shape
Head and tail - best known spider like
Bacteriophage lambda
Lambda discovered by Ester Lederberg in 1950
Viruses use their hosts molecular machinery to reproduce
Developmental programme with diff proteins needed at diff stages
So diff genes expressed at diff times e.g. lysis gene expressed last to pop cell - must not be done early
Bacteriophages are bacterial viruses.
They coordinate expression of genes to facilitate transition through stages
Early genes come on a few minutes after infection coding proteins needed early and late gene regulators
Early genes stall late gene expression making sure that processes like lysis do not occur too early
Phage Lambda injects DNA into host
DNA is either
1) incorporated into host chromosome - lysogeny
Or
2) used to make new phage particles - lytic process - host cell is burst/killed
Molecular switch allows phage to asses and decide which to do. Decided by
- how many phage in bacterium
- food availability
- health of host cell
Lysogeny
Phage DNA integration into chromosome producing a prophage hybrid chromosome. Integrated lambda DNA replicates with host chromosome
- having phage DNA in the bacterium’s chromosome prevents further bacteriophage infection - can be considered beneficial
