18: Bacterial Transcription Flashcards
What is the RNA World Theory?
before last common ancestor, there was no DNA; RNA made proteins
Why is there are RNA step in gene expression? (3 reasons)
- The RNA step provides an amplification which contributes to differential gene expression
- Since RNA can be degraded, expression of a gene can be stopped quickly
- RNA provides additional opportunities to regulate gene expression (especially in eukaryotes during processing and export from nucleus)
What is a consensus sequence?
The most frequent base at each position in a group of functionally related DNA elements
there is a consensus sequence of all DNA/RNA binding for a protein
tight binding for most similar sequence to consensus sequence
What is a promoter?
the DNA sequence required to initiate transcription of a gene or operon
What is a terminator?
the DNA sequence required to stop transcription
What is an operon?
a set of bacterial genes transcribed from a single promoter and thus expressed from a common RNA
set of genes that are transcribed together
only bacteria have operons, but not all bacteria have operons
What is the operon structure?
- transcription start site (+1) (can be any nucleotide)
- promoter before +1 (-1 region)
- transcript (protein encoding genes)
- terminator
What are the key features of a Bacterial Promoter?
- ~100 bp
- 2 consensus sequences (highly conserved)
- -35 region and -10 region
- the rest is random bases
How were the -10 and -35 sequences found?
- comparing the sequences of many E. coli promoters
- Shared elements suggest functional importance = consensus
What are the characteristics of the -10 region?
- TA rich
- TA = less energy to separate 2HB by RNA poly
How can you test if the -35 sequence is important?
make mutations:
normal: 100%
mutate: 2%
Why are promoter seuqneces exactly idendical to consensus?
control amount of RNA transcript: less similar to consensus = less transcript = less energy waste
What is Bacterial RNA Polymerase - RNAP?
- enzyme that makes RNA transcripts using DNA as a template and nucleoside triphosphates (NTPs) as substrates
What is the RNAP structure? What are its characteristics?
- core enzyme made of 4 protein subunits that assemble and stay together (B, B’, w, a)
- can make RNA but can’t recognize promoters
- promoter specificity of RNA poly is detmined by SIGMA SUBUNIT which recognizes and binds to -35, -10 region in promoter sequence
- sigma subunit sometimes disassembles
polymerase core + sigma subunit = RNA polymerase holoenzyme
PROTEIN-PROTEIN INTERACTION
How does sigma unit recognize sequence?
fits well with promoter, stalls as it moves also DNA