The E. coli bacterium prefers to thrive on glucose but it is not always available. When glucose runs out, there is an increase in cyclic AMP, which binds to catabolite activator protein or CAP. This binds to the promotor region of the lac operon, which increases RNA polymerase activity and causes lac operon transcription. This is an example of an activator protein acting on the lac operon. Lactose needs to be present and glucose need to be diminished. Only when both conditions are present will transcription levels of the lac operon be high. Prokaryotes have the ability to sense oncoming stress by releasing alarmones, which are small nucleotide derivatives. They can stimulate the expression of stress-related genes. There are certain virulence genes that can be upregulated in response to alarmone production. There are transcription factors in eukaryotes as well. Proteins called enhancers can bind a distance away from the gene they regulate, with DNA looping putting the enhancer and promotor together. There are bending proteins that allow for DNA bending, which can put the enhancer in proximity to the promotor. There is also what’s called epigenetic regulation, which involves methylation of certain base pairs in order to affect transcription. Usually, the methylation of cytosine will often slow transcription. Histone proteins can also affect transcription.
GENETIC DIVERSITY IN PROKARYOTES Vertical gene transfer involves the passing on of genetic information from a mother cell to daughter cells. This works differently when comparing sexual organisms and asexual organisms. Mutations will add to gene diversity in prokaryotes and eukaryotes. Asexual reproduction usually involves passing on direct copies of the parent genome. Prokaryotes can engage in what’s called horizontal gene transfer, which involves the passing on of genetic material in the same generation. It can occur between species that are not directly related to one another. There are three ways that this can occur. In transformation, naked DNA is taken up from the outside environment. In transduction, there is a viral vector that transmits DNA. In conjugation, pili are used to transfer DNA from one cell directly to another cell. Figure 49 shows these processes in a bacterial cell:
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