The Life Cycle Of A Virus

Close your eyes and look. What you saw at first is there no more; and what you will see next has not yet come to life. —Leonardo da Vinci
We can apply these words very aptly to a virus—of the bacteria-infecting kind known as bacteriophage. When a phage particle enters a cell, it loses its infective power and its identity as a particle. Generally its entrance into the cell is followed within 15 minutes to an hour by the emergence of a new generation of infectious virus particles. Sometimes, however, there is no immediate pathological event. The genetic material of the virus that has passed into the cell combines with the genetic material of the cell itself. In doing so it is converted into something that has been named a 'provirus,' meaning before virus. Days or years afterward the provirus may suddenly develop into virus and the bacterium give rise to a group of virus particles.
The term provirus needs some explanation. The expression 'proman' would certainly not evoke the idea of a human egg, from which Homo sapiens always develops, but rather that of an evolutionary ancestor of man which would have to undergo a genetic transformation to become man. A provirus may perhaps correspond to an evolutionary ancestor of a virus. But it is also much more than that.
Before attacking the question of the nature of proviruses, we must know something about viruses themselves. What is a virus? We shall leave out of the discussion the much debated issue as to whether viruses are living organisms or not; our concern is to find out how they differ from 'normal' organisms of the microbiologist's world. The two attributes that are usually thought to define viruses are their very small size and the fact that they can multiply only inside living cells—usually requiring a specific kind of cell host. But to learn more about their peculiarities let us go beyond this definition and compare viruses with other small biological units.
First of all, how does a virus differ from a cell? Most cells are capable of reproducing themselves: they possess the genetic material which is the basis of heredity and the tools necessary to synthesize the essential building blocks and to organize these into a structure just like themselves. We can see three important differences between a cell and a virus, taking bacteriophage as a typical virus: (1) whereas cells contain both desoxyribonucleic acid (DNA) and ribonucleic acid (RNA), the phage contains only DNA; (2) whereas cells are reproduced from essentially all their constituents, bacteriophage is reproduced from its nucleic acid; (3) whereas cells are able to grow and to divide, the virus particle as such is unable to grow or to undergo fission. Bacterial viruses are never produced directly by division of an existing virus; invariably they are formed by organization of material produced in the host cell.
Next we must consider whether viruses bear any likeness to the particles within a cell, particularly the particles called plasmagenes. Here differences are less easy to find. The theory has been proposed that viruses may originate as mutated plasmagenes. But we know that some plasmagenes (e.g., the chloroplasts of green plants) can grow and divide. Furthermore, plasmagenes are not pathogenic or lethal to the cell, as virus particles are. Let us just note, for the time being, that nothing which resembles a bacteriophage in its properties, life cycle, shape or organization has been found in normal cells.
Let us now consider the peculiar behavior of the virus. The virus particle as such is only the beginning and the end of a life cycle. Its only physiological function is to obtain entry of the virus' genetic material into the host cell. After this occurs, what remains of the virus is devoid of infectious power. There follows a vegetative phase in which the specific constituents for new viruses are produced. Finally these constituents are organized into virus particles, which are liberated by lysis (dissolution) of the cell. The whole process usually takes from 15 to 60 minutes.
But a bacterial virus may multiply in another way, and this is where the provirus enters the picture. Ordinarily the virus nucleic acid passed into a cell proceeds promptly to multiply and to synthesize specific protein material for new phage particles. Sometimes, however, the nucleic acid may anchor onto a bacterial chromosome and act as if it were a normal constituent of the cell. It behaves as a bacterial gene, being replicated at each bacterial division and transmitted to each daughter bacterium. This is what we call a provirus. It is a potential virus; it may eventually give rise to virus particles. In the meantime the bacterial offspring go on growing and dividing as normal bacteria, and each daughter bacterium yields progeny capable of producing viruses. In other words, the ability to produce viruses is perpetuated inside the bacterium; no new infection from outside is needed.
Bacteria containing proviruses are called lysogenic. When a small number of such bacteria are broken down, no infectious particles can be found. This means that the provirus is not infectious. And yet in every large population of these bacteria some mature bacteriophage particles appear. From time to time a bacterium in such a culture suddenly disappears, and about 100 phage particles emerge. The probability of a lysogenic bacterium spontaneously giving rise to viruses varies from 1/100 to 1/100,000. In some systems the probability is apparently independent of external factors; it cannot be modified. In other lysogenic systems phage production may be initiated at will by inducing agents, such as X-rays, ultraviolet rays, nitrogen mustard and other substances—all of which are known to be capable of producing mutation. Within 30 to 60 minutes after exposure to one of these agents, practically all of the bacteria produce viruses and lyse.
How do lysogenic bacteria produce viruses? Before discussing this question we must know more about the proviruses. We are inclined to think that proviruses originally arose as mutants of normal bacterial genes. Whatever their origin, the reservoir of bacterial viruses seems to be the provirus-carrying bacteria. These bacteria may have perpetuated provirus, that is to say, the hereditary ability to produce virus, for many thousands of years.
The study of lysogenic bacteria has led to a clear picture of the provirus. Apparently it does not contain virus protein, for lysogenic bacteria do not cause the production of specific antibodies to phage protein in experimental animals. It is therefore tempting to visualize the provirus as a large molecule of nucleic acid. Secondly, the provirus is associated with a certain genetic character of bacteria and is located at a specific site on a bacterial chromosome. Thirdly, two genetically related proviruses in a bacterium may cross over and recombine. Fourth, a lysogenic bacterium is immune to infection by a phage particle related genetically to its provirus, though it can be killed by an unrelated phage. As long as the provirus remains in that state, a genetically related superinfecting phage is unable to develop into phage. Finally, the mere presence of the provirus may modify the properties of a bacterium: it may endow certain bacteria with the ability to produce a toxin they could not otherwise make, or it may change the typical appearance of bacterial colonies. Things happen as if the provirus either carries a specific gene or modifies the neighboring bacterial genes. From all these data it may be concluded that provirus is the bacterial virus' genetic material, bound to a specific site in the bacterium and responsible for a specific bacterial immunity.
Now it is difficult to imagine that this immunity is due only to the presence of the provirus. A particle cannot exert a specific action by its mere presence. The only way the provirus can make the bacterium immune—that is, prevent multiplication of a virus invader—is by modifying or blocking a specific activity of the bacterium necessary for that reproduction. And the provirus can do this only if it is present at a specific site. As a matter of fact, we can account for all the properties of proviruses and of lysogenic bacteria by the hypothesis that the provirus is the genetic material of the virus anchored at a given site in the bacterium. The genetic material of an infecting virus becomes a provirus when and because it becomes bound at that site to a specific receptor, which modifies the material. It then gives the bacterium immunity against genetically related infecting particles. An inducing agent such as ultraviolet rays destroys the immunity because it displaces the genetic material of the virus from its specific site.
For a long time virologists have concentrated on the virus particle itself. Yet the particle is only a prelude to the infection. During the longest and most important part of the life cycle, the pathogenic phase in a cell, no virus particle is present. As a matter of fact, disappearance of the virus particle is the sine qua non for the development of the cellular lesion. Indeed, there are cases in which all the bacteria in a lysogenic population die although very few of them produce bacteriophage particles; the cells are killed by a defective development of proviruses initiated by an inducing agent. One could even conceive of a condition in which the probability of the virus ever appearing would be infinitely small, that is to say, practically absent. Some bacteria actually carry a gene which can initiate the synthesis of a protein lethal to themselves. But that is another story.
Biologists have long been accustomed to think of death in terms of the destruction or alteration of some vital structure. We have been less inclined to think of living cells as carrying the seeds of their own destruction, or of the possibility that lethal agents may kill in more than one way. For example, X-rays sometimes kill by destroying essential structures, but they may also destroy a cell by inducing a gene to express its lethal potentiality. This potentiality is sometimes the power to start a new synthesis which may or may not end in virus particles.
To what extent are the phenomena disclosed in bacteria valid for higher organisms? May animal or plant cells perpetuate proviruses? Are some viral diseases of man the result of the activation of a provirus? May immunity to virus diseases be explained in terms of proviruses? Do the findings concerning lysogeny have any bearing on cancer? Let us recall that the inducing agents which can trigger proviruses to give rise to viruses are all not only mutagenic but also carcinogenic—radiations, nitrogen mustard and so on. It is indeed tempting to theorize that carcinogens may induce malignancy by initiating the formation of a pathological structure from a provirus-like material. Many facts are in favor of the hypothesis that proviruses originate some animal diseases, but the problem cannot be discussed within the limits of this article. Suffice it to say that this is, at any rate, a working hypothesis.
I have tried to outline the concept of the provirus, to analyze its relations with the concept of the cell and of the virus and to show the impact of the newly acquired knowledge on our conceptions of cellular disease. The common denominator of the various phases of the life cycle of a virus is the genetic material—the nucleic acid—which may exist in three states: infectious, proviral and vegetative. Throughout these three states the genetic material apparently remains essentially the same in structure, but it changes radically in dynamic potentialities and behavior. The virus particle, the end product of the vegetative phase, is a quiescent nucleoprotein particle, unable to grow or to divide. The provirus is an integrated nucleic acid, which behaves like a gene and is replicated like the host genes. Neither the virus particle nor the provirus is pathogenic per se; their pathogenic property is only potential. The only pathogenic phase of the virus is the vegetative phase, during which the specific viral nucleic acid multiplies and during which the specific viral protein is synthesized. Things happen as if the synthesis of the protein is responsible for pathogenicity.
The provirus produces provirus; it is order. The vegetative particle produces virus particles and a disease of the host; it is disorder. The virus particle does not produce anything; it is an extremely conservative particle—the absence of any activity, that is to say, a kind of order. Thus the virus is an alternation of order and disorder.
As a result, my presentation of the subject may seem somewhat disordered. For this I had decided to apologize, when I came across an unpublished letter which Martin de Barcos, Abbot of Saint-Cyran, wrote to Mother Angélique in 1652: 'Allow me to tell you that you would be wrong to apologize for the disorder of your discourse and of your thoughts, because, if they were otherwise, things would not be in order, especially for a person belonging to your profession. As there is a wisdom which is folly before God, there is also an order which is disorder, and in consequence, there is a folly which is wisdom and a disorder which is the true rule.' This being exactly the case of the virus, I decided not to apologize.