Regulation of Transcription (lac
Much of the original work deciphering how
transcription is regulated was done
on the lac operon. The 1965 Nobel
Prize in Medicine was awarded for this research. Bacteria often organize genes in groups with similar
functions called operons. RNA polymerase binds to the promoter at the 5'
end of the operon and transcribes the genes into RNA. The 2006 Nobel
Prize in Chemistry was awarded for research on the structure and regulation
of RNA polymerase.
Bacteria need to be very efficient, and only express enzymes when they are
needed. An example is in metabolism of lactose, a disaccharide found in
milk. Bacteria will express the enzyme beta-galactosidase the highest when
they only have lactose as an energy source. If other energy sources such
as glucose are present, expression of beta-galactosidase will be
decreased. Finally, if no lactose is present at all, beta-galactosidase
will not be expressed.
Transcription and Translation
For a bacteria to express the genes on the lac operon they are first
transcribed by RNA polymerase. The regulation of the expression of most
genes is done at the level of transcription. The transcribed mRNA is then translated
into proteins by ribosomes.
Response to Lactose
E. coli express a protein called the lac
repressor. In the absence of lactose, this protein can bind to DNA
near the promoter for the lacZ gene. This prevents RNA polymerase from
binding, and no transcription is possible.
If lactose is present, the lactose binds
to the lac repressor, inducing a conformational change that causes the repressor
to fall off of the DNA and transcription can proceed at a fairly low level..
Response to Glucose
Low levels of glucose levels in a cell lead to the formation of cyclic AMP (cAMP).
cAMP binds to a protein called CAP
or CRP. Once CAP is bound to cAMP, it can bind to a region upstream of
the promoter, where it accelerates RNA polymerase binding. This leads to
increased transcription of the lacZ gene.
Eukaryotic genes contain Exons (which encode information that will end up
being translated into protein) separated by Introns (sequences that will not
encode proteins). At the 5' end of a gene we find a promoter and often a
CpG island. Both of these elements regulate the transcription of a
gene. At the 3' end of a gene we find a stop sequence, and a signal for
Transcription is the process of copying the genetic information in DNA into RNA. This is done by the enzyme RNA polymerase which binds to a region called a promoter found at the 5´ end of a gene. The binding of RNA polymerase is tightly regulated by many proteins called transcription factors.
RNA polymerase transcribes the genomic DNA sequence into the corresponding RNA
sequence resulting in the formation of a primary transcript.
In Eukaryotes this primary RNA transcript is then processed into a messenger RNA (mRNA) if the gene is to be translated into protein. This involves removing introns, which do not encode for protein, and
splicing the remaining exons together. At the 5´ end of the mRNA a 7-Methyguanosine residue is added to provide a protective cap. At the 3´ end of the mRNA, 50-100 adenosine residues are added, generating a poly A tail.