MODERN BOTANY - P. RAVEN - 1990

SECTION III. GENETICS

CHAPTER 8. CHEMISTRY AND HEREDITY

Regulation of Gene Transcription

Gene METABOLISM/31.html">Transcription is under meticulous cellular control. Even in Bacteria, certain switching systems operate to regulate the transcription of specific genes at the right time and to the necessary extent. Eukaryotes apparently share many of the capabilities found in bacteria, although the fine details of the mechanism regulating gene transcription in eukaryotes are less well understood.

Thanks to intensive research on several bacterial genes, we know a great deal about how their transcription is regulated. Perhaps the best-studied mechanism is lactose metabolism in the bacterium Escherichia coli. Normally, this bacterium does not encounter the disaccharide lactose and therefore does not synthesize the Enzymes required for its metabolism. If lactose is added to the culture medium, the bacteria begin to synthesize large amounts of the enzyme β-galactosidase, which splits the disaccharide into glucose and galactose (Fig. 8-11).

Class="center">Fig. 8-11. In the bacterium Escherichia coli, The breakdown of a lactose molecule into galactose and glucose requires the enzyme β-galactosidase. β-Galactosidase is an inducible enzyme, meaning its production is regulated by an inducer—in this case, the substrate, the disaccharide lactose.

Thus, the lactose substrate causes, or induces, The production of the enzyme required for its breakdown. There are many known instances of such enzyme synthesis being induced by an energy-rich substrate. In other cases, an enzyme involved in the synthesis of a particular amino acid or other metabolite is not produced in The Cell in the presence of that amino acid; this repression of enzyme synthesis by key metabolites utilizes the same control system as induction.

The Operon

François Jacob and Jacques Monod, working in France, developed the operon concept to explain how bacterial Cells regulate enzyme Biosynthesis. For this research, they were awarded the Nobel Prize in 1965. An operon is a group of structural genes located along a single segment of DNA; it serves as a unit of Translation, and the expression of all its genes is coordinated because they are transcribed together to form a single mRNA molecule. For example, the lac operon of E. coli contains the gene for β-galactosidase and two other genes whose products are involved in lactose metabolism.

The simultaneous action of the enzymes encoded by all the GENES OF THE lac operon is necessary for the bacterium to utilize lactose. In each cell growing on a medium containing lactose, about 3,000 molecules of β-galactosidase are found, accounting for approximately 3% of the total cellular protein. In the absence of lactose, expression of the lac operon genes does not occur because a specific protein called a repressor blocks it. The repressor binds to DNA at a specific site, specifically between the genes themselves and the region where RNA polymerase attaches to the DNA. When lactose enters the cell, it binds to the repressor protein, altering its shape in such a way that it loses its affinity for DNA. This allows the polymerase to move unimpeded along the DNA, transcribing the lac operon genes that encode the enzymes.

Operons—that is, groups of linked genes transcribed together to form a single mRNA molecule as in bacteria—have not been found in eukaryotes. Each cytoplasmic mRNA molecule in a Introduction/5.html">Eukaryotic Cell carries information for the synthesis of only a single protein. However, regulatory systems do occur in plants. Mutations manifesting as the unrestricted production of groups of enzymes have been discovered in corn (Zea mays) and evening primrose (Oenothera).

Feedback Inhibition

In addition to genetic mechanisms that alter cell Functions by controlling the synthesis of individual enzymes, A number of physiological control systems operate through direct feedback inhibition—in other words, by controlling enzyme activity.

Finely tuned regulatory systems, such as end-product inhibition, serve as Examples of allosteric interactions. The binding of a specific molecule (an allosteric effector) to a particular site on a protein can sufficiently influence the weak interactions that determine its tertiary Structure (see Chap. 3) to cause a change. When a protein molecule undergoes conformational changes, the region that enables the protein to function as an enzyme may be affected and altered. Such interactions can control the level of specific cellular enzyme activity, thereby influencing the functional activity of the cell. The binding of lactose to the repressor protein discussed above leads to allosteric changes in this protein. In some cases, the allosteric effector may be the end product of the metabolic pathway that begins with the reaction catalyzed by the allosteric enzyme.

Control of Plant Development

The genetic processes we have discussed are ultimately responsible for the expression of individual Organism traits. However, the establishment of traits characteristic of an adult organism is invariably preceded by differentiation and development. Differentiation is the process by which unspecialized cells or Tissues gradually change, begin to perform specific functions, and acquire a characteristic structure. Development refers to the Organization OF THE complex set of tissues that form the adult organism. The adult organism is represented by a complex assembly of many different tissues that maintain specific physiological and morphological relationships with one another. Ultimately, all of them originate from a single cell. In diploid organisms, this is the fertilized egg, or zygote. As it develops, increasingly complex tissues are organized, which also requires more rigorous control.

The developmental processes of animals and plants differ radically. Animal development follows a definite plan in a precise sequence; order and temporal coordination are decisive factors in achieving the final result. At each stage, strict Control of Gene Expression is exercised. In animals, external conditions generally do not strongly influence the course of development; once the adult state is reached, development ceases.

In plants, by contrast, the developmental process is continuous: plants develop throughout their entire lives. Their development is directly influenced by both external factors (which have a minimal effect in animals) and internal ones. Tissues-specific plant differentiation is governed by Hormones whose synthesis is sensitive to environmental changes (see Chap. 24). The ability of plants to respond to external factors AIDS in their adaptation to specific habitats—an important factor given the inability of plants to "seek out" more favorable conditions by moving from place to place.

In plants, almost any differentiation is reversible, provided the differentiated cell is alive, The Nucleus has been preserved within the protoplast, and a secondary wall has not formed. This accounts for the absence of a rigid developmental plan in plants. In animals, such reversibility is quite rare.

The reversibility of plant development can be illustrated by experimental data. German botanist Gottlieb Haberlandt suggested as early as 1902 that all living plant cells are totipotent, meaning that each possesses the potential ability to become any other plant cell. Haberlandt believed that a single piece of plant tissue or even an isolated cell was capable of forming an entire adult plant, but he was unable to accomplish this practically. More than half a century passed before this hypothesis was definitively proven.

The problem essentially boiled down to not knowing which substances needed to be added to the growth medium. In the 1950s, Cornell University plant physiologist F. C. Steward isolated a small piece of carrot (Daucus carota) ROOT phloem and placed it in a liquid culture medium in a rotating flask. The medium contained sucrose and mineral elements necessary for plant growth (see Chap. 26), as well as certain Vitamins (organic substances that the plant cannot synthesize). However, some additional substances were required for growth and differentiation. Steward discovered them in coconut milk, which is known for its rich content of growth substances, although their nature had not yet been elucidated at the time.

In the rotating flask, individual cells continuously detached from the growing cell mass and floated freely in the medium. These single cells were capable of growing and dividing to form small clusters. Long before, Steward had noticed that many of these new cell clusters formed roots. If the cells were left in the rotating flask, they ceased to differentiate, but when transferred to a solid medium (Agar in these experiments), shoots formed in some of the clusters (Fig. 8-12). After being transplanted into soil, the small plants became covered with leaves, flowered, and produced seeds.

Fig. 8-12. Two buds formed on undifferentiated tissue (callus) of a geranium after Treatment with two hormones—auxin and cytokinin. The calluses of certain plants continue to grow either as undifferentiated tissue or as roots or buds, depending on the hormone ratio.

The presented results indicate that at least some Cells of the differentiated phloem tissue contain the complete Genetic information required to develop an entire plant, even though it remains unexpressed. Furthermore, the experiments demonstrate that such differentiated cells are capable of expressing parts of their previously unexpressed genetic material when appropriate signals are received from the environment. (The hormonal control of this differentiation pathway is discussed in Ch. 24.) Having obtained these findings, Steward finally corroborated Haberlandt's hypothesis.

The form and structure of an adult organism are determined by the program encoded within its genes. However, the traits of such an organism are fully manifested only through complex processes of differentiation and development. These processes are controlled by the interaction of products translated during the organism's development.

The Influence of Cytoplasm on Differentiation

In many organisms, cytoplasmic components play a direct role in Cell Differentiation. These include Organelles such as Plastids and Mitochondria, which contain their own DNA (see Ch. 2). If these organelles are distributed unevenly between daughter cells during Cell Division, The Fate of subsequent cell "generations" may vary significantly. This accounts, in particular, for many of the "maternal effects" well known in genetic literature—differences between identical hybrids caused by the Influence of the maternal organism.

Similarly, chemical gradients—that is, variations in substance concentration across different PARTS OF THE cell—play a crucial role in differentiation. For example, in the brown alga Fucus (see Fig. 15-9,5), a gradient of stored insoluble nutrient particles is apparently established in the zygote under the influence of gravity or, perhaps, electrochemical forces. This gradient determines THE POSITION OF the spindle During the first division of the zygote and, consequently, the "fate" of the two cell generations originating from this division. Unequal cell division can be vital for the distribution of various cytoplasmic elements and the determination of cell Lineage fates.

In both animal and plant cells, numerous substances continuously diffuse at varying rates in different directions, and distinct tissue types are frequently located in close proximity to one another. Therefore, the effect observed in Fucus may also be identified in other, simpler organisms. Extremely fine-tuned control over development can result not only from substance gradients within individual Cells and Tissues; the differentiation of any cell can be largely determined by its position within the body of a developing plant or animal. Certain pathways of interaction involving hormones and other factors in plant development are discussed in Sec. VI.