By: Meryl Liu
Prokaryotic gene expression is significantly different from that of eukaryotic organisms, mainly due to major differences in gene organization. A cistron is the smallest unit of genetic material that can transmit a distinct amount of genetic information, and in prokaryotes, most DNA and the messenger-RNA that is transcribed is polycistronic (encodes for two or more proteins in multiple open-reading frames, usually sequentially for a single metabolic pathway), while eukaryotes are typically monocistronic and have greater separation between enhancer sequences and the genes that are regulated. A good example of polycistronic gene organization and regulation of gene expression in prokaryotic organisms, especially bacteria, are operons, a cluster of related genes that are co-transcribed or repressed while sharing the same source of regulation (usually a single signal or substrate).
As all the structural genes coding for certain proteins and enzymes in an operon are needed for a single biochemical metabolic pathway at a certain time (i.e. the breakdown of lactose by E. coli), operons consist of a certain structure: Promoter, Repressor, Operator, and Genes. One can remember this structure through the mnemonic PROG. The promoter region is the area in which RNA polymerase binds in order to synthesize an mRNA strand, but transcription factors influence its ability/rate to do so. However, the repressor is a transcription factor that stands in the way of RNA polymerase and suppresses transcription by binding to the Operator region of the DNA unless a certain substrate either binds to the repressor and causes it to the leave the operator (Inducible) or causes the repressor to bind to the operator and stop the transcription of the genes (Repressible).
An example of a repressible operon is the trp operon, which is responsible for the synthesis of tryptophan, an important amino acid. However, typically when there is low environmental tryptophan, the operon is turned “on” and the repressor is not bound to the operator, thus allowing for the structural genes to be transcribed and tryptophan to be synthesized. When tryptophan is accumulated and is present at a higher concentration in the cell, two tryptophan molecules act as substrates that bind to the repressor, changing its shape and causing it to bind to the operator which turns the operon off, stopping RNA polymerase from transcribing the genes and synthesizing more tryptophan than what is already needed.
On the other hand, the most common inducible operon is present in E. coli and is known as the lac operon, producing three enzymes needed for the catabolism of lactose (milk sugar) into glucose and galactose. When lactose is not present in the cell, the repressor binds to the operator and turns it off, but when lactose is present and available to be processed for energy, lactose in the cell is converted to allolactose, which is an inducer molecule that binds to the repressor and removes it from the operator, allowing RNA polymerase to begin transcribing the structural genes. The ability of bacteria to break down lactose in the intestinal tract causes symptoms present in humans with lactose intolerance, who are unable to produce the lactase enzyme needed to break down lactose.
The development of the prokaryotic model for gene regulation by Jacob and Monod in the early 1900s has made important advances in the understanding of genetic circuitry, cellular control, and even heredity in epigenetics, specifically in the area of cancer research in eukaryotic models. This, in turn, led to the conceptualization of “reversion” in cancer: reverting malignant cells to their normal state without altering specific DNA sequences. In 1961, molecular biologists Pitot and Heidelberger outlined a hypothesis in which, rather than understanding cancer as a direct result between a carcinogen and genetic material interaction, a carcinogen actually binds to a repressor and impacts the cellular growth process, making cancer phenotypic and possibly reversible. The lac operon served as a major basis for this theory, which continues to be tested today as a revolutionary perspective on the genetic basis of cancer.
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Oehler, S, et al. “The Three Operators of the Lac Operon Cooperate in Repression.” The EMBO Journal, U.S. National Library of Medicine, Apr. 1990, www.ncbi.nlm.nih.gov/pmc/articles/PMC551766/.
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Pitot, Henry C., and Charles Heidelberger. “Metabolic Regulatory Circuits and Carcinogenesis.” Cancer Research, American Association for Cancer Research, 1 Nov. 1963, cancerres.aacrjournals.org/content/23/10_Part_1/1694?iss=10%2BPart%2B1.
Q: How is genetic material organized differently between prokaryotes and eukaryotes?
A: In eukaryotic organisms, gene organization is monocistronic, meaning that within a single open reading frame, only one major protein or mRNA sequence can be transcribed at a time. In prokaryotes, however, gene organization is polycistronic: a specific group of enzymes or proteins needed for a metabolic pathway, for example, is transcribed in tandem by a sequence of multiple cistrons (independent units of genetic information).
Q: What is the difference between an inducible operon and a repressible operon?
A: An inducible operon is turned “on” when there is a large amount of substrate present in the cell: enough to pass a threshold and cause the operon to be transcribed. An example of this is the lac operon in E. coli, which codes for 3 important enzymes needed in the breakdown of lactose. A repressible operon is turned off when there is a large amount of substrate present in the cell; an example is the trp operon. When tryptophan becomes low in the cell, the trp operon is turned “on,” and codes for the synthesis of tryptophan.