The regulation and expression of gene activity

The regulation and expression of gene activity

The regulation of gene activity is best controlled at the level of gene transcription. Many examples of such regulation have been discovered in E. coli and other bacteria.

Recall that in the bacterial chromosome the genes controlling the enzymes of a metabolic pathway are adjacent to each other. Several adjacent genes code for a single, long mRNA molecule that directs the synthesis of several enzymes of a specific metabolic pathway. The consequence of such an arrangement is that the amount of synthesis of gene products is coordinately regulated. Therefore, if a cell is stimulated to synthesize a large amount of one of the enzymes of a group, it will also make large amounts of the other enzymes of the same group. This kind of regulation involves the induction and repression of enzyme synthesis at the gene level and was discussed. Maintenance of induction requires the continued synthesis of mRNA to balance its degradation. Thus this mRNA instability, coupled with transcriptional control, assumes that only necessary proteins are synthesized by the cell.

We can better understand the regulation of gene expression in prokaryotes by discussing the lactose (lac) operon of E. coli. This operon is now by far the best-understood part of any cellular genome. There are other bacterial operons, but they are less well understood and are different in detail from the E. coli lac operon.

The lac operon

When inducers such as lactose or other are added to a culture of e. Coli, there is a 1,000- fold increase in the rate of synthesis of the enzymes (which hydrolyzes lactose to glucose and galactose), permease (which transports lactose into cell), and thiogalactoside transacetylase (which plays no role in lactose utilization but may play role in detoxifying certain thiogalactoside). The genes for these proteins are linked together on the e. Coli chromosome. In the absence of control, the rate of enzyme production would be constant and depend only on the structural genes (such as z, y, and a), amino acid levels, activating enzymes, and other substances. However, the control of the rate of enzyme synthesis is directed by the regulator genes designated I, p, and o where I am the repressor gene, p the promoter gene, and o the operator gene. The I gene codes for a repressor protein which binds to the DNA of the operator o gene, thus preventing transcription, that is the synthesis of mRNA. The promoter gene p is considered to be the site on the DNA where the RNA polymerase enzyme, catalyzing the synthesis of mRNA, binds, and is thus the site where the specific lac mRNA (responsible for the biosynthesis of the specific enzymes of the operon) synthesis begins. Let us discuss the functioning of the Jacob-Monod model of gene control for the lac operon as it is now understood.

1. Genes function as templates or blueprints for the transcription of mRNA, using the protein-synthesizing machinery of the cell (ribosome), the mRNA directs the synthesis of polypeptides ( the long chain of amino acids) in a process called translation.
2. The genes z, y, and a operate as a single unit of transcription, which is initiated at p.
3. Transcription of the operon is both negatively and positively controlled.

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Fig:The lac operon

Negative control is mediated by the lac repressor which binds to the o gene and blocks transcription. Inducers, such as lactose, stimulate lac mRNA synthesis by binding to the repressor and reducing its affinity for the operator. Both repression and induction of enzyme synthesis are negative control systems because, in either case, the synthesis of the enzyme can proceed only when the repressor is removed from its blocking site on the o gene.

Positive control of enzyme synthesis is said to occur when an association between a protein and a part of the regulatory region of an operon is essential for expression of related structural genes in the operon. Expression of the lac operon is inhibited when a more efficient source of energy, such as glucose, is present in the medium. The presence of glucose results in a decreased concentration of intracellular cyclic AMP (adenosine-3’,5’-monophosphate). Cyclic AMP is necessary for efficient expression of the lac operon since it activates the catabolite gene activator protein (CAP), which in turn activates transcription of lac mRNA by RNA polymerase at the promoter site.

Thus both cyclic AMP and a specific inducer acting in concert are necessary for the synthesis of many inducible enzymes in e. Coli. The Little enzyme is made if either is absent.

The Jacob-Monod model of the lac operon has given biologists an insight into the molecular events of gene regulation. It shows up impressively the precision by which regulatory proteins modulate gene function: the repressors must recognize the specific nucleotide sequences of the operator gene on the one hand, and the other hand they must recognize specific inducer molecules like lactose. An understanding of such regulatory mechanisms has been extended into the study of bacterial viruses.

Upon entering a bacterial host cell, the DNA genome of phage may either proceed to a developmental cycle leading to host-cell lysis or integrate into the chromosome of the host bacterium, making it lysogenic or temperate. Several phage genes are involved in deciding how fast a critical level of a specific repressor can be produced. When sufficient repressor is available, it blocks transcription of all the other phage genes by combining with two separate operators that control two important operons. In such a circumstances, no phage proteins are made, the host cell does not lose, and the circular DNA is capable of being integrated into the host chromosome. If the repressors are destroyed or inactivated at any time during lysogency of the bacterial cell, the phage operons become depressed and start functioning the genome replicates, and the cell lyses. Thus it is seen that the repressor gene controls the fate of both the bacterial cell and the bacteriophage.

The mechanisms of the operon model of the gene regulation are applicable only to prokaryotic organisms and viruses. The same mechanisms have not been found to occur in eukaryotic cells in which the situation is more complex. For example, animal and human genes are full of “gibberish”: segments of DNA that serve as coded genetic instructions are interrupted by other segments that have no function whatever. These extraneous pieces of DNA are called introns and often make up a larger portion of the gene than the actual code-bearing sequences (called exons). Thus the introns must all be spliced out of the genetic message before the cell use it. It has been suggested that in eukaryotic cells regulation must involve controlling the functioning of the mRNA rather than its synthesis; that is, translation is controlled rather the transcription.

Furthermore, since cells with the same genome function differently in different organs in multicellular eukaryotic organisms, there must be some means for switching on and off whole sets of genes in particular cells. Since the chromosome of eukaryotic cells does not exhibit any clustering of functionally related genes, any mechanism for controlling transcription must act, directly or indirectly, on many genes distant from each other. That is, there is coordinate control of many genes in different chromosomes.

In addition, it has been found that the mRNA that codes for a polypeptide does not always have the full sequence of nucleotides of the corresponding gene. Such mRNA have been modified, after transcription, by splicing with specific enzymes.

Our knowledge of the mechanism of gene control in eukaryotic organisms is still fragmentary. But progress is slowly being made.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.