To confer the information stored in DNA into
functional molecules such as RNA and proteins, a large amount of energy is required. Therefore, gene expression is strongly regulated. Via this regulation, the gene product activity
of mainly proteins is also controlled. This enables cells to respond to environmental
changes, for example, a change in the nutrient supply. The first fully described genetic regulatory
mechanism is the lac operon in E. coli bacteria. Today, it still represents an adequate model
for prokaryotic gene regulation. An operon is a transcription unit of genes
whose products are required under identical circumstances. So, it facilitates the coordinated expression
of multiple genes. The DNA sequence of an operon comprises three
different components: a promoter, an operator, and several genes, each of which codes for
a protein. In the absence of lactose, a repressor protein
is bound to the lac operator. This binding prevents transcription of the
downstream lac genes. The repressor protein is encoded by the regulatory
gene lacI. lacl isn’t directly part of the lac operon but is located a few base pairs
upstream. As soon as lactose is present in the environment,
it’s taken up by the bacterium. Lactose then binds in the form of allolactose
to the permanently expressed repressor protein. This binding inactivates the repressor, unblocking
the operator. Now, the RNA polymerase can bind to the promoter
and read the subsequent genes. This process is termed substrate induction,
since it can only occur after the substrate lactose enters the cell. The three genes lacZ, lacY, and lacA are now
transcribed together as a polygenic mRNA. Three different proteins are synthesized on
this mRNA, namely β-galactosidase, permease, and transacetylase. These proteins are essential to lactose metabolism. Permease forms pores in the bacterial cell
membrane, facilitating further lactose uptake into the cell. The enzyme β-galactosidase breaks down lactose
to simple "sugar residues" that can then be metabolized. An additional special mechanism is used in
the presence of lactose and absence of glucose. Glucose is usually the preferred energy source
for E.coli. In the presence of glucose, lactose degradation
is possible but not essential for survival, and proceeds as just described. However, if there’s a very low glucose concentration,
the cell needs to break down as much lactose as possible. This is secured as follows:
If little glucose is available in the cytosol, the cAMP concentration increases. cAMP then binds to the catabolite activator
protein, in short CAP. This cAMP-CAP complex forms a dimer that binds
to the DNA close to the lac promoter, thereby increasing RNA polymerase activity. As a result, the three enzymes involved in
lactose metabolism are synthesized at a higher rate, allowing the breakdown of more lactose,
compensating the glucose deficiency. Through their concentration, both lactose
and glucose affect the gene expression of enzymes involved in lactose metabolism. This enables rapid adaptation to different
environmental conditions. In bacteria, genes are regulated by operons. Although the regulation of gene expression
in eukaryotes is considerably more complex, it’s based on the same concepts as, for
example, the use of activator and repressor proteins.