Positive Control
Positive control helps RNA polymerase to transcribe a gene through the use
of regulatory proteins called "activator proteins". In the active
state, activator proteins bind to DNA near the promoter region, and helps
RNA polymerase bind to the promoter region so that it can transcribe that
gene. It is thought that most promoters that require an activator protein
do not fit RNA polymerase well. The role of the activator protein is to
alter the shape of the DNA (by twisting or bending it) so that RNA polymerase
fits the promoter better. In the inactive state, the activator protein cannot
bind to DNA, and the gene is not transcribed.
Maltose Induction
E. coli can also grow using the sugar maltose as a carbon and energy source.
The enzymes required to utilize maltose are also induced by the presence
of maltose. In this case, however, maltose binds to and activates a positive
regulatory protein (an activator protein). When maltose is present, the
activator protein can bind to DNA near the promoter region for the maltose
utilizing genes. This activator alters the DNA in such a way that allows
RNA polymerase to bind, and transcription can begin.
In the absence of maltose, the activator protein cannot bind to DNA, and
synthesis s of the maltose enzymes stops.
This is still induction, because the presence of the small molecule, maltose,
causes the maltose utilization enzymes to be made. However, in this case,
induction is carried out through the use of an activator protein.
Leucine Repression
Activator proteins can also be used to repress enzyme synthesis. In the
case of leucine, another essential amino acid, the presence of leucine shuts
off the transcription of genes required to make leucine from scratch. Only
when there is no leucine around does E. coli then make the enzymes required
for leucine biosynthesis.
This is accomplished with an activator protein that only binds DNA when
there is no leucine in the cell. When leucine is present, some leucine binds
to the activator protein, causing it to change to the inactive conformation.
If the activator protein cannot bind to DNA, then RNA polymerase cannot
begin transcription of the leucine biosynthesis genes.
Now that you've read about it, watch the positive control animation!