Gene Expression Control: Prokaryotes and Eukaryotes
Control of Gene Expression in Prokaryotes
The repressor produced by the regulatory gene is associated with the operator area and prevents RNA polymerase from transcribing the structural genes. This operon functions as a system of enzyme induction, in which some molecules called inducers (in this case, lactose) are associated with repressors, causing changes in their structure. Consequently, the repressors lose affinity for the operator area, and RNA polymerase transcribes the structural genes. Other operons function according to this system of repression by the final product, such as the operon that regulates the synthesis of histidine. In it, the repressor can only bind to the operator if it is associated with histidine.
Control of Protein Biosynthesis by Cyclic AMP
In addition to the operon model, another type of control called regulation by cyclic AMP (cAMP) has been discovered. This molecule is formed from ATP by the action of the enzyme adenylate cyclase, located on the inner surface of the plasma membrane. cAMP requires the action of the catabolite activator protein (CAP). The CAP-cAMP complex has an affinity for the promoter region, anterior to the place where the RNA polymerase lies. It seems that in the absence of the complex, RNA polymerase, which causes the mRNA for the enzymes that metabolize lactose, has many difficulties associating with the promoter. When the level of glucose in the cell increases, the level of cAMP decreases because glucose, upon crossing the cell membrane, is converted to glucose-6-phosphate. This conversion consumes ATP, making it unavailable to form cAMP. Without enough cAMP to form the CAP-cAMP complex, RNA polymerase is not fixed and will not produce the enzymes for lactose metabolism. Only when glucose is exhausted, and in the presence of lactose, do bacteria produce new enzymes that metabolize lactose.
Control of Gene Expression in Eukaryotes
The cells of multicellular eukaryotic organisms with differentiated tissues respond to hormonal changes in the internal environment. Although all cells have the same DNA, they do not express the same information. This is the cause of cell differentiation. The DNA segments that are highly condensed are not expressed, while those that are transcribed are extended. Cell differentiation occurs according to the areas that are condensed, leading to the formation of organs in the embryo. Each type of cell has different membrane receptors, and therefore, only some may be targets for certain hormones. The control of gene expression due to hormones differs depending on the type of hormone:
- Lipid Hormones: Due to their composition, lipid hormones readily cross the plasma membrane. In the cytoplasm, they bind to intracellular receptor proteins, forming a hormone-receptor complex, which is directed to the nucleus. It binds to certain DNA sequences and induces the transcription of certain genes, probably facilitating the decondensation of DNA in some areas. For example, anabolic hormones cause muscle protein synthesis.
- Protein Hormones: Due to the size and nature of their molecules, protein hormones cannot pass directly through the plasma membrane. To do so, they bind to specific proteins on the membrane, forming a hormone-receptor complex. This process activates the enzyme adenylate cyclase, which converts ATP to cAMP, known as the second messenger (with the hormone being the first messenger). cAMP is directed to the nucleus and activates transcription regulatory proteins. The hormone does not penetrate the cell but, by contacting the membrane, stimulates the formation of cAMP.