Prokaryotic Gene Regulation
If all cells have the same DNA content, and the DNA of a cell
specifies its activities (what enzymes it makes) and characteristics
(what effect these enzymes have), why aren't all cells the same? We
know, however, that there are different cell types in our bodies, and
that the activities of these cells changes with time. The
hormone-producing cells in the pituitary gland only produce growth
hormone during childhood and adolescence. These same cells remain in
the pituitary in adulthood, but they don't function to produce growth
hormone. How do they know when they are needed or not needed?
This question as it applied to large, compex organisms like humans was
very daunting for scientists in the first half of the 20th
century. Francois Jacob and Jacques Monod approached the problem from
a more basic and simple perspective. How did single-celled prokaryotes
like E. coli know how to respond to their environments? Each
environmental cue generates a specific response, with specific
proteins and reactions. For example, a bacterium can use several
different sources of nitrogen. Some bacteria can incorporate diatomic
nitrogen gas from the air, or incoprporate ammonia from their
surroundings, or break the amine group from the end of an amino acid
like glutamine. It is much easier and less energy costly for the cell
to use the nitrogen from glutamine than to fix nitrogen gas from the
air. These two processes rquire very different enzymes to allow them
to occur. If there is glutamine around, the cell should be able to
shut off the enzymes that are involved in incorporation of nitrogen
gas. In fact, it shouldn't have to waste the energy to synthesize
these enzymes at all. How can the cell "turn off" the synthesis of
proteins from its DNA, when the moment calls for it?
In all these cases, the cell has to have some way of shutting off the
unwanted protein selectively and leaving on the other genes in the
cell. As you can imagine, in terms of energy cost, it is better to
shut off the process as early as possible, so that no energy is wasted
in mRNA and protein synthesis. This type of early-intervention control
is called transcriptional regulation, since expression of the
gene is regulated at the level of mRNA synthesis, or transcription.
- The cell could somehow selectively inhibit transcription of the gene. The mRNA for this gene would never be made.
- The cell could selectively degrade the mRNA as soon as it was made, preventing it from being translated into protein.
- The cell could selectively prevent translation on an otherwise stable mRNA.
- The cell could selectively degrade the translated protein so that it couldn't waste energy trying to catalyse reactions that the cell has no need for at that time.