Viruses and Evolution

VIRUSES AND EVOLUTION E. A. EVANS, JR.* As recently as 1942, Julian Huxley could write: "Bacteria (and a fortiori viruses ifthey can be considered to be true organisms) . . . appear to be not only wholly asexual but premitotic. Theirhereditaryconstitution is not differentiated into specialized parts with different functions. They have no genes in the sense ofaccurately quantized portions ofhereditary substance and therefore they have no need for the accurate division of the genetic system which is accomplished by mitosis" (1). It is an index ofthe progress ofour knowledge that we now know that not only bacteria but viruses do indeed have genes and that, further, the discovery ofa variety ofreproductive mechanisms in bacteria in addition to simple binary fission has disclosed striking similarities to the genetic processes in other organisms. A corresponding increase in our information concerning molecular structure, the mechanism ofhost-cell invasion, and the process ofreproduction has occurred also for a number ofviral agents (2, 3). All ofthis, however, is not yet sufficient to offer a definitive answer to the problem ofthe specific evolutionary status ofviruses in the hierarchy of living organisms. Even so, it becomes clear that viral particles have biological properties other than their occasional role as infectious and destructive agents. Documentation of this statement requires a summary of the contemporary information regarding viruses. I. All viruses contain, minimally, nucleic acid and protein, although other organic and inorganic components may be present (polyamines, lipids, coenzymes). The nucleic acid may be either deoxyribonucleic or ribonucleic acid. This is in marked contrast to all other known forms of living cells, where both DNA and RNA are present, although the * Department ofBiochemistry, University ofChicago. This paper was prepared for the Darwin Centennial Celebration held at the University ofChicago, November 24-28, 1959. 213 DNA is concentrated mainly in the nuclear structures while the RNA exists predominantly in the cytoplasm. In the absence oftheir hosts, viral particles such as the bacteriophages, tobacco mosaic virus, influenza, and poliomyelitis viruses do not show demonstrable metabolic activity, do not contain energy "reservoirs," and do not require any source ofenergy to maintain their structure. It appears therefore that we are dealing with structures which, although enormous in terms of molecular size, are bound by the usual covalent bonds of organic substances together with such other types ofintramolecular binding forces as operate, for example, in determining the particular configuration ofa protein molecule. 2. The viral protein is apparently physiologically heterogeneous and, in the case ofthe bacterial viruses, continuing examination has disclosed an increasing number of different roles for viral protein in addition to its being responsible for viral antigenicity. In general, the viral protein appears responsible: a)For the protection of the nucleic acid component, since the intact virus particles are not attacked by enzymes that split nucleic acids. b)For the specific introduction ofviral nucleic acid into the host cell; or, conversely, for excluding viral nucleic acid from non-susceptible cells. The first case is illustrated by coliphage T2, where viral invasion involves (i) preliminary combination with the bacterial cell and eventual alteration ofthe structure ofthe protein ofthe viral tail piece; (ii) subsequent erosion of the host cell wall adjacent to the attached virus by a lytic enzyme (protein) present in the viral tail; and (iii) contraction ofstill another protein in thevirus tailpresumably to facilitate the entrance ofthe viralnucleic acid into the host cell (4). The second property of the viral protein has been demonstrated in both bacterial and animal virus infections. Removal ofthe distal portion ofthe virus tail from coliphage T2 permits the attack of host cell membranes impervious to the intact virus. With human cells, the ribonucleic acid from poliomyelitis virus is capable ofinducing and causing viral replication in tissues not attacked by the original virus particle with intact protein component (5). The organ and tissue specificity ofanimal viruses may be, then, a reflection of the nature ofthe protein component ofthe infectious particle. c)For causing, in some but not all cases, the death of the host cell in the absence ofviral replication. This can be demonstrated with nucleic acid-free "ghosts" of the T-even coliphages (6). The lethal effect of 214 E. A. Evans, Jr. · Viruses and Evolution Perspectives in Biology...