For example, the physical properties of DNA and RNA are remarkably identical in all organisms, and these are perhaps easiest to study in bacteriophage systems.
Bacteriophages, or phages for short, are viruses that parasitize bacteria. Viruses are an extra ordinarily diverse group of ultramicroscopic particles, distinct from all other organisms because of their noncellular organization.
Composed of an inert outer protein shell, or capsid, and an inner core of nucleic acid—either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) but never both—viruses are obligate intracellular parasites, depending to a great extent on host cell functions for the production of new viral particles.
There is considerable variation in size and complexity among viruses. Some have fewer than ten genes and depend almost entirely on host functions. Others are known to contain from thirty to one hundred genes and rely more on proteins encoded by their own DNA.
Even the largest viruses are too small to be seen under the light microscope, so studies on viral structure rely heavily on observation with the transmission electron microscope.
The Study of Bacteriophages
Because scientists know more about the molecular and cell biology of the common bacterium Escherichia coli than about any other cell or organism, it is perhaps not surprising that the best-known phages are those that require E. coli as a host (coliphage).
It is not possible to observe phage growth directly (as bacterial growth can be detected by the appearance of colonies on an agar plate), but phage growth can be indirectly observed by the formation of plaques, small clear areas in an otherwise continous lawn of host bacteria growing on a solid growth mediumin a petri dish.
Bacteriophages can multiply by two different mechanisms, termed the lytic cycle and the lysogenic cycle. Some phages are capable only of lytic growth, while others retain the ability to reproduce by either lytic growth or entry into the lysogenic cycle. In the lytic cycle, phages first attach themselves to specific receptor sites on the host cell wall.
The phage nucleic acid (DNAor RNA) is injected inside the host, while the protein capsid of the infecting particle remains outside of the host cell at all times. Once the DNA or RNA is inside, transcription of phage genes begins, and phage-encoded proteins begin to be made.
Some of these proteins serve to inactivate and destroy the host cell DNA, ensuring that the cell’s energy resources will be directed exclusively toward the production of phage proteins and the replication of phage nucleic acid. Phage DNA or RNA replication ensues quickly and is followed by the packaging of this genetic material into the newly synthesized capsids of the progeny phage particles.
The final step is host cell lysis—the bursting of the host cell to release the completed and infective phage progeny. The number of phages released in each burst varies with growth conditions and species, but ideal conditions often result in a burst size of one hundred to two hundred per host cell.
For temperate bacteriophages, those capable of entering the lysogenic cycle, infection of the host cell only rarely causes lysis. Injection of the phage DNA into the host is followed by a brief period of messenger RNA (mRNA) synthesis, necessary to direct the production of a phage repressor protein, which inhibits the production of phage proteins involved with lytic functions.
A DNA-insertion enzyme is also made, allowing the phage DNA to be physically inserted into the DNA of the host. The cell then can continue to grow and multiply, and new copies of the phage genes are replicated every cell generation as part of the bacterial chromosome.
The host cell is said to be lysogenic, for it retains the potential to be lysed if the prophage pops out of the host DNA and enters the lytic cycle. The integrated prophage does confer a useful property on the host cell, however, for the cell will now be immune to further infection from the same phage species.
One of the best-known lytic phages, which is often used in genetic studies, is the coliphage T4. Its protein capsid consists of three major sections—the head, the tail, and the tail fibers.
The double-stranded circular DNA molecule of T4 is packaged into the icosahedral-shaped head, and during the infection process it is forced through the hollow core of the cylindrical tail and then directly into the host cell. Contact with the cell is established and maintained throughout the infection process by the tail fibers.
Self-assembly of progeny phages occurs in at least three distinct cellular locations, as complete heads, tails, and tail fibers are first assembled separately and then pieced together in one of the last phases of the infection cycle.
Packaging of the replicated T4 DNA is an integral part of the head assembly process. Each of the three subassemblies involves a reasonably complex and highly regulated sequence of assembly steps.
For example, head assembly is known to require the activity of eighteen genes, even though only eleven different proteins are found as structural components of mature heads. Identification of the number and sequence of genes involved with each subassembly process has been facilitated by the analysis of artificial lysates from t8 mutants.
For those temperate phages capable of entering the lysogenic cycle, many additional strategies for genetic control and regulation have evolved. The most thoroughly studied of the temperate coliphages is phage lambda (λ).
Genes controlling phage DNA integration, excision, and recombination, and those involved with repressor functions, have been identified in phage λ as well as structural genes involved with lytic functions that are similar to those studied in T4.
One of the most important conclusions to be drawn from studies on bacteriophages, and viral genetics in general, is that many of the results have universal implications. For example, the physical properties of DNA and RNA are remarkably identical in all organisms, and these are perhaps easiest to study in bacteriophage systems. The experiment that provided the final proof that DNA was the genetic material was performed using a coliphage very similar to T4.
Studies on the origin of spontaneous mutations, first performed in phage, have extended to higher forms of life as well. Some of the most basic questions concerning protein-DNA interactions are best addressed in viral systems, and the principles that emerge seem to hold for all other experimental systems.
There is every reason to believe that many basic questions in cell and molecular biology will continue to be best studied in viruses such as bacteriophages, and that some of these investigations will spawn applications that can directly benefit humankind.
It is certain that advances in molecular biology that have revolutionized the understanding of cell biology and the molecular architecture of cells will continue to expand the frontiers of knowledge in the study of viral genetics. Applications in human medicine, veterinary medicine, and plant breeding are sure to follow, as scientists continue to unravel the complexities of these simplest of organisms.