Download control of gene expression

Control of Protein expression
•Transcription is most regulated step in gene expression in prokaryotes
•Successful survival requires adaptability and economy:
•In prokaryotes mRNA is generally very short lived (a few minutes)
•Much of the regulation of what proteins are expressed is at the
transcriptional level in prokaryotes
Points of transcriptional control
• Accessibility of chromatin
• Promotor sequence and how conserved it is (affects
RNA polymerase binding)
• -10 and –35 sequences and how conserved
• Sigma factors
• Whether or not a repressor protein is present
• Enhancer/activator sequences
• Once the transcript has been produced there is the
opportunity for anti sense RNAs to get involved in control
e.g. by binding to sense message and preventing it from
being translated
• mRNA processing and turnover
• large transcripts are processed into smaller units as a
means of regulating the expression of specific genes
within each operon.
Chromatin
DNA in the bacterial chromosome is condensed through part of the cell cycle.
This can affect its availability for transcription.
Promoters
The nucleotide sequence of promoters is similar but not identical. The more
similar the sequence is to a consensus sequence, the more likely that RNA
polymerase will attach and produce mRNA from the associated genes.
Sigma factors
Different sigma factors recognize different promoters thus, the availability of
sigma factors can regulate the transcription of genes associated with these
promoters.
The availability of sigma factors can be used to regulate sets of genes.
E.g., a group of genes whose product is rarely needed might have a different
promoter sequence than other genes and thus require different sigma
factors. These genes would only be transcribed when the correct sigma
factor became available.
Repressors and activators
See Lac and tryp examples.
Some activators in particular can act on a sequence a long way away from
gene and promoter.
Activation at a distance
Synthesis of antisense RNA
• mRNA is synthesized off the template
(antisense) strand of DNA. Antisense RNA
is synthesized from the noncoding (sense)
strand of DNA. The two mRNA molecules
bond together, inactivating the mRNA
mRNA Processing and turnover
1. RNA polyadenylation
• Post transcriptional additions of As to 3’ end of RNA strand
• Growth rate dependent
• 20-50 nucleotides added
• Not all RNA types are adenylated
• Results in decreased transcript stability
• Enzymes involved include
• polyA polymerase 1 (PAP 1)
• polyA polymerase 2 (PAP 2)
• PAP1 is responsible for 90% of polyadenylation
• RNA molecules are adenylated with different degrees of efficiency
– Presence of specific ss segments at either 5’ or 3’ end thought to be important
•
How polyadenylation effects/facilitates degradation
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In proks most messages end with a hairpin (due to transcription termination)
3’ end would not normally be accessible to ribonucleases as they act on ss RNA
Addition of 3’ tail makes ss piece and allows nucleases access
RNA degradation is performed by multicomponent complex called an RNA
degradosome
Proteins
involved in degradosome include
•
PNPase (an exonuclease)
•
RNase E
•
Enolase and an RNA helicase
•
Catalytic RNA
•
PAP 1 is thought to be an accessory factor
•
As a result of getting the poly A tail the RNA molecule becomes accessible to the
exonucleases which munches it up to ultimately mononucleotides. The enolase and
helicase are involved in unwinding the stem loops
2. mRNA stability
• Affected by presence of hairpin loops and
stem loops
• More secondary structure more stability
• Seems to be because degrading enzymes
can not easily access