This article is concerned with the use of viral models for the study of the mechanism of protein biosynthesis and its regulation. The scope is restricted mainly to general aspects of animal viral systems and how these systems may be used to approach the question of cellular regulation. Most information on the regulation of metabolic processes in eukaryotic cells comes from the study of bacteria and from the successful application of this knowledge to higher systems. However, differences in regulation of the translation of genetic information from the messenger RNA into protein may be expected between prokaryotes and eukaryotes. Due to the short half-life of prokaryotic mRNAs, transcription has been considered as the main mechanism controlling gene expression. Nevertheless, during recent years firm evidence has been accumulated for additional regu latory factors operating during translation. This topic was recently reviewed by HASELKORN and ROTHMAN-DENES (1973) and by KOZAK and NATHANS (1972)."

"When we give a definition it is for the purpose of using it." HENRI POINCARE in Science and Method A. Objectives The first version of this paper was written to introduce new students and fellows of my laboratory to the mysteries of herpesviruses. Consonant with this design sections dealing with well documented data were trimmed to the bone whereas many obscure phenomena, controversial data and seemingly trivial observations were discussed generously and at length. There is some doubt as to whether it was meant to be published, but it was not a review. The objective of reviews is frequently to bring order. But alas, even the most fluent summation of credible data frequently makes dull reading and too much plausible order, like very little entropy in chemical reactions, is not the most suitable environment on which to nurture the urge to discover. This version is more charitable but not less inbalanced. The bibliography reflects the intent of the paper and was updated last in December of 1968. It should be obvious without saying that no single account such as this can do justice or injustice, as the case may be, to the several hundred papers published on herpesviruses each year or to the many thousand papers published on herpesviruses since the first of the members of the family was experimentally transmitted to a heterologous host more than half a century ago (GRUTER, 1924). B. Definition 1.

The study of streptococcal infections and their sequelae has in the last two decades yielded several important findings on the biological properties of cellular and extracellular products of group A streptococci. These findings have contributed to a better knowledge of the pathological reactions occurring in the macroorganism during host-parasite interactions. Nevertheless, the pathogenesis of streptococcal infections is not fully understood. So far there has been no success in eliciting experimentally, either through the action of the substances isolated from the cell, or from broth culture filtrate of group A streptococci, symptoms that are fully identical with any type of acute streptococcal infection. It also has not been possible to explain the mUltiplicity of clinical and histological changes caused by streptococci as being due solely to anyone of these substances or a combination thereof. The same applies to the sequelae of streptococcal infections, rheumatic fever and acute glomerulonephritis. We do not know how the group A strepto coccus elicits these diseases and we have only a partial understanding of the pathological processes, initiated by this streptococcus, and resulting in cardiac or renal lesions. It is clear that an organism infected by streptococci is exposed to the action of a complex of substances. A more detailed recognition of the biological activity of the single components and their combination under defined experimental conditions may be capable, it is hoped, to explain the pathologic processes triggered in the course. of the development of group A streptococcal infection."

Ever since arbovirus infections became known and their relative importance assessed, experiments were designed to elucidate the mode of transmission and the most important natural hosts responsible for perpetuating the infection in nature. Human infections and the disease in wild rodents, birds, and domestic animals were studied in relation to viremia and distribution of the infectious agent in the organism. With increasing epidemiological studies it became apparent that the neural manifestations of the disease are very uncommon, confined only to a small percentage of individuals of the most susceptible species. Various factors have been proposed to explain why in certain instances the virus becomes establish ed in the central nervous system and causes a serious or lethal disease. For example, differences in the virulence of the virus strains, varying susceptibility of individuals of one species, or intercurrent circumstances facilitating access of the virus to the central nervous system were alleged. Also, various possible routes of entry of the virus into the brain and spinal cord have been considered."

Several discoveries are noteworthy for allowing us to probe the recesses of the virus infected cell and to search for cryptic viral genomes which might provide clues in our studies of cancer etiology or developmental biology. One of the most notable was the dis covery of reverse transcriptase. This marked a momentous occasion in the history of molecular biology. Not only did it provide insight into the mechanism of persistence of retroviruses but it also provided us with an enzyme that could synthesize a DNA copy of any RNA. This DNA copy could then be used as a hybridization reagent to search for both complementary DNA and viral-specific RNA. Thus one could follow the course of any viral infection or probe in tumor cells for hidden viral genomes. Second, a great deal of credit must be given to the geneticists who isolated the various deletion mutants in the 'avian retrovirus system and thus provided us with the frrst means of isolating gene-spe cific probes. Finally, the laboratories which have mapped the genome have provided us with the framework in which to ask very specific questions with our gene-specific probes. Recently, numerous excellent reviews concerning various aspects of the retroviruses have appeared. In this review I shall not even attempt to present a comprehensive review of retroviruses."

General aspects of nucleic acid uptake by mammalian cells have been the subject of several reviews during the last few years (PAGANO, 1970; BHARGAVA and SHANMUGAM, 1971; DUBES, 1971; RYSER, 1967). These reviews covered methods used for the infection of cells by viral nucleic acids as well as interaction of mammalian cells with non-viral nucleic acids. This article is restricted to a discussion of experiments with poliovirus RNA and focuses special attention on the steps following the uptake of RNA into a cell, aspects that were not discussed in earlier review articles. The fate of input RNA once inside the cell is determined by the host cell but experimental conditions can be chosen to favor the survival of input RNA and the induction of a virus growth cycle by interfering with host-cell meta bolism through events that, in the case of infection with intact virus, might be controlled by viral proteins."

This volume is dedicated to the memory of the late Professor WERNER BRAUN, one of the most devoted and active members of the Editorial Board of the Current Topics in Microbiology and Immunology, who passed away, after suffering a heart attack, in November 1972. Dr. WERNER BRAUN was born in Berlin, Germany, on November 16,1914. During his highschool days in Berlin he did research work on problems of genetics as a young guest in the Kaiser-Wilhelm-Institut fur Biologie, in the department of Prof. R. GOLDSCHMIDT. I remember his colourful description of his discussions during this period, while still a teen-ager, with OTTO WAR- BURG. He studied biology and medicine at the University of G6ttingen and received a Ph.D. degree in biology in 1936. In the same year he left Nazi Germany and came to the United States first as a Guest Investigator in Genetics at the University of Michigan at Ann Arbor, and then in Berkeley, where he carried out his work in the Depart- ments of Zoology and of Veterinary Science until 1948. He was engaged during this period in the study of problems concerned with physiological genetics, bacterial variation, immunology and biochemistry.

1. 1 Scope of the Review This review was intended initially as a reference source for those interested in the origins and fITst descriptions ofthe defective avian sarcoma viruses. Quite a few of these viruses have been characterized in the past few years and their varied nomenclature according to source, discoverer, date of isolation or biological properties could result in some con fusion among those attempting to follow the literature. Information will be included on the molecular biology of the sarcoma viruses, rather more of which is available than when the review was fITst conceived, although in this respect the review will inevitably be out of date by the time of publication. If any bias of content is introduced, this will be towards a more detailed coverage of the author's own area of interest, the gene products of the defective sarcoma viruses. Rous sarcoma virus (RSV) serves as the model for much of this work and will frequently be referred to for comparative purposes, as will the mammalian defective transforming viruses. Recent reviews provide more complete coverage of these topics (l-4a). This review is complemented by a discussion ofthe avian acute leukaemia viruses, which appears elsewhere in this volume 5]. As will be seen, concentrating primarily on the defective sarcoma viruses and comparing them to RSV can be justified in terms of their biochemical properties as well as their similar biology."

The expression of many bacterial genes adapts itself in an almost in stantaneous and reversible way to specific environmental changes. More specifically, the concentration of a number of metabolites, a function of the amounts of enzymes involved in their synthesis or degradation, in turn retroacts on the rate of synthesis of these enzymes. The genetic bases for this regulation were established by JACOB and MONOD (1961). These authors also showed how the known elements of these regulatory mechanisms could be connected into a wide variety of circuits endowed with any desired degree of stability, in order to account for essentially irreversible processes like differentiation (MONOD and JACOB, 1961). The general principles used by JACOB and MONOD in their study of negative regulation were extended to positive regulation by ENGLESBERG et al. (1965). An independent approach permitted the discovery of positive controls in temperate bacteriophages (see below, III). Each control operation is mediated by a pair of complementary genetic elements (hereafter called "control cell"): a control gene which produces a l control (or regulator) protein and a control site which is the target for the regulator protein. Negative control means that the control protein (repressor) prevents gene expression. One deals with positive control when the control protein (activator) is necessary for this expression. It has become apparent that, as initially postulated by JACOB and MONOD, control of gene expression operates, at least to a large extent, at the transcriptional level.

Phenomena as diverse as tuberculin sensitivity, delayed sensitivity to soluble proteins other than tuberculin, contact allergy, homograft rejection, experimental autoallergies, and the response to many microorganisms, have been classified as members of the class of immune reactions known as delayed or cellular hypersensitivity. Similarities in time course, histology, and absence of detectable circulating immunoglobulins characterize these cell-mediated immune reactions in vivo. The state of delayed or cellular hypersensitivity can be transferred from one animal to another by means of sensitized living lymphoid cells (CHASE, 1945; LANDSTEINER and CHASE, 1942; MITCHISON, 1954). The responsible cell has been described by GOWANS (1965) as a small lymphocyte. Passive transfer has also been achieved in the human with extracts of sensitized cells (LAWRENCE, 1959). The in vivo characteristic of delayed hypersensitivity from which the class derives its name is the delayed skin reaction. When an antigen is injected intradermally into a previously immunized animal, the typical delayed reaction begins to appear after 4 hours, reaches a peak at 24 hours, and fades after 48 hours. It is grossly characterized by induration, erythyma, and occasionally necrosis. The histology of the delayed reaction has been studied by numerous investigators (COHEN et al. , 1967; GELL and HINDE, 1951; KOSUNEN, 1966; KOSUNEN et al. , 1963; MCCLUSKEY et al. , 1963; WAKSMAN, 1960; WAKSMAN, 1962). Initially dilatation of the capillaries with exudation of fluid and cells occurs.