Gene Regulation and Protein Levels
Cells are able to live in diverse and complex environments. They respond to nutrients, toxins, signaling molecules from other cells and physical parameters like temperature and osmotic pressure. They can also sense internal signals like DNA damage (to start a repair mechanism) and key metabolites.
To respond to the described conditions the cell uses molecules called transcripton factors. Transcirption factors have the ability to eather physically block the RNA polymerase from binding to the DNA sequence near the promoter called operator site, in this case it's a repressor that represses the binding and decreases the transcription rate, or the transcription factor enhances the affinity for the RNA polymerase and therefore increases the transcription rate, such a transcription factor is called activator. Each transcripton factor is activated by its signal (e.g. the sugar lactose) from the environment. The signal pushes the equilibirum of the transcription factor, which is constantly changing between its active and inactive state, to the active state. The active transcripton factors can bind DNA to enhance the rate of transcription by increasing the affinity for the RNA polymerase. The RNA polymerase in turn produces more RNA and thus more protein is produced. Through this mechanism the protein level can be controlled by the signalling molecule. In E. coli, there exist about 300 transcription factors that regulate about 4000 genes.
Sometimes the CDS contains more than one gene. A group of genes that is regulated by the same promoter is called Operon. Genes on an Operon are expressed polycistronic. This means only one strand of mRNA is formed which in turn encodes multiple proteins. An example is the Pathway needed to produce tryptophan. All necessary Proteins are expressed polycistronic.