Theme 3: Prokaryo.c Transcrip.onal
Regula.on
19.3 Transcrip.onal Regula.on In Prokaryotes
• Gene Expression- The func+onal product of the gene is made, modified
and ac+vated
◦ Three levels of regula+on must occur:
• Transcrip+onal control to allow for the transcrip+on of DNA
to mRNA
▪ Controls the amount of messenger RNA produced in
the cell
▪ Undertaken by controlling the binding of proteins to
the promoter
▪ Slowest
• Recall that the cell is star+ng from scratch
▪ OIen prevalent with more dras+c environmental
changes that a cell can be exposed to
▪ Most efficient and economical for the cell
• Transla+onal control to allow for the transla+on of mRNA to
proteins
▪ Controls the amount of protein produced
▪ Ini+a+on of transla+on in eukaryotes occurs by
binding of the ribosome to 5' end of mRNA (i.e. 5' cap)
▪ In prokaryotes, the ribosome will bind to and ini+ate
transla+on at specific Shine-Dalgarno sequences
• Post-transla+onal control to allow for modifica+ons and
ac+va+on of produced proteins
▪ Allow polypep+de chain to be folded into a func+onal
3D structure
▪ Includes the driving of the protein into complexes, the
binding of substrates or the unmasking of enzyma+c
domains
▪ Fastest
, • Once the cell receives an appropriate signal, this
can lead to a simple modifica+on to turn on all
the inac+ve protein stored up from the previous
process
19.3.1 Transcrip.onal regula.on can be posi.ve or nega.ve.
• Posi+ve Regula+on: A regulatory molecule (usually a protein) must bind
to the DNA at a site near the gene in order for transcrip+on to take place
◦
◦ The binding site for the ac.vator may be upstream of the
promoter (as in the above diagram) or downstream of the
promoter or even overlap the promoter
◦ Some+mes, the ac+vator protein combines with a small molecule
and undergoes a change in shape that alters its binding affinity for
DNA
• Change in shape is a type of allosteric effect
• Genes subject to this type of posi+ve control usually encode
proteins needed when the small molecule is present
▪ Example: In E. coli, the genes for the degrada+on of
the sugar arabinose are regulated by an ac+vator that
binds to the sugar; When arabinose is present, genes
are transcribed; When arabinose is absent, the genes
aren't transcribed
◦ For some genes, ac+vator proteins can bind to DNA only when
small molecule is absent
• These genes usually encode proteins needed for synthesis of
the small molecule
• Example: In E. coli, the genes for synthesis of the amino acid
cysteine
• Nega+ve Regula+on: A regulatory molecule (usually a protein) must bind
to the DNA at a site near the gene in order for transcrip+on to be
prevented
◦ What turns transcrip+on off is binding with a repressor protein
• The repressor may be upstream from, downstream from or
overlapping the promoter
Regula.on
19.3 Transcrip.onal Regula.on In Prokaryotes
• Gene Expression- The func+onal product of the gene is made, modified
and ac+vated
◦ Three levels of regula+on must occur:
• Transcrip+onal control to allow for the transcrip+on of DNA
to mRNA
▪ Controls the amount of messenger RNA produced in
the cell
▪ Undertaken by controlling the binding of proteins to
the promoter
▪ Slowest
• Recall that the cell is star+ng from scratch
▪ OIen prevalent with more dras+c environmental
changes that a cell can be exposed to
▪ Most efficient and economical for the cell
• Transla+onal control to allow for the transla+on of mRNA to
proteins
▪ Controls the amount of protein produced
▪ Ini+a+on of transla+on in eukaryotes occurs by
binding of the ribosome to 5' end of mRNA (i.e. 5' cap)
▪ In prokaryotes, the ribosome will bind to and ini+ate
transla+on at specific Shine-Dalgarno sequences
• Post-transla+onal control to allow for modifica+ons and
ac+va+on of produced proteins
▪ Allow polypep+de chain to be folded into a func+onal
3D structure
▪ Includes the driving of the protein into complexes, the
binding of substrates or the unmasking of enzyma+c
domains
▪ Fastest
, • Once the cell receives an appropriate signal, this
can lead to a simple modifica+on to turn on all
the inac+ve protein stored up from the previous
process
19.3.1 Transcrip.onal regula.on can be posi.ve or nega.ve.
• Posi+ve Regula+on: A regulatory molecule (usually a protein) must bind
to the DNA at a site near the gene in order for transcrip+on to take place
◦
◦ The binding site for the ac.vator may be upstream of the
promoter (as in the above diagram) or downstream of the
promoter or even overlap the promoter
◦ Some+mes, the ac+vator protein combines with a small molecule
and undergoes a change in shape that alters its binding affinity for
DNA
• Change in shape is a type of allosteric effect
• Genes subject to this type of posi+ve control usually encode
proteins needed when the small molecule is present
▪ Example: In E. coli, the genes for the degrada+on of
the sugar arabinose are regulated by an ac+vator that
binds to the sugar; When arabinose is present, genes
are transcribed; When arabinose is absent, the genes
aren't transcribed
◦ For some genes, ac+vator proteins can bind to DNA only when
small molecule is absent
• These genes usually encode proteins needed for synthesis of
the small molecule
• Example: In E. coli, the genes for synthesis of the amino acid
cysteine
• Nega+ve Regula+on: A regulatory molecule (usually a protein) must bind
to the DNA at a site near the gene in order for transcrip+on to be
prevented
◦ What turns transcrip+on off is binding with a repressor protein
• The repressor may be upstream from, downstream from or
overlapping the promoter
