The Evolution of Viruses - Zhdanov, V. M. 1990

Special Section
Adenoviruses

Adenoviruses comprise a large group of Viruses that infect warm-blooded animals. Their genome Structure and virion Morphology distinguish them from other viral groups. Adenoviruses are classified into the family Adenoviridae, which includes two genera—mammalian and avian adenoviruses—Mastadenovirus and Aviadenovirus [Dreizin R. S., 1982; Norrby E. et al., 1976; Matthews R., 1982]. Further subclassification within both genera is based on serovars and host species: human adenoviruses hl — h41, cattle bos1 — bos9, pigs susi — sus4, sheep ovil — ovi5, horses equl, dogs can1 — can2, goats capri, mice musi, chickens gall—gall9, turkeys mell—mel2, geese ansi, pheasants phal, and ducks anal. Human adenoviruses are divided into 5 subgenera, whose representatives differ in fiber length, Sequence Homology degree, Molecular Weight of internal Proteins, etc. Based on their oncogenic properties in rodents, adenoviruses are divided into group A: highly oncogenic (serovars 12, 18, and 31), moderately oncogenic (3, 7, 11, 14, 16, 21, 34, and 35), and low-oncogenic (the remaining serovars). In tissue cultures, virtually all adenoviruses are capable of cellular transformation. Attempts have been made to correlate high oncogenicity with a low G+C content.

The Genome consists of a double-stranded DNA with a molecular weight of approximately 23.8×10⁶. The DNA length is 11 µm, with a terminal protein covalently attached to the 5' end of the molecule. Adenoviral DNA in virions is circular, and the terminal proteins interact with each other non-covalently. The DNA ends contain terminal inverted repeats of 100–140 Base Pairs. Consequently, upon removal of the adenoviral terminal protein molecules, the DNA forms circular structures through base pairing. The terminal protein is involved in DNA circularization. Adenoviral DNA is infectious; infectivity increases sharply in the presence of the terminal protein.

Virions have a buoyant density of 1.33–1.34 g/ml in a cesium chloride density gradient, an icosahedral Symmetry, and a diameter of 70–90 nm. They consist of 252 capsomeres with a diameter of 8–9 nm, including 240 hexons and 12 penton bases, to which club-shaped fibers terminating in a knob are attached. The DNA is located within the capsid cavity, associated with core protein, and folded into loops.

Virions contain 11–15 Polypeptides designated by Roman numerals from II to XII: hexon Peptides (II), penton base (III), its associated protein and fibers (IIIa, IV), core proteins (V, VII), and hexon-associated proteins (VI, VIII–XII). The molecular weights of the capsid proteins are 120,000 (II), 24,000 (VI), 13,000 (VIII), 19,000 (IX), 85,000 (III), and 62,000 (IV), while those of the core proteins are 48,000 (V) and 18,500 (VII). Core proteins are histone-like. Protective proteins include hexons, pentons, and fibers, with the latter being associated with type-specific Immunity.

Adenovirus Replication proceeds in stages: adsorption, virion penetration into The Cell and partial deproteinization, early syntheses, METABOLISM/36.html">DNA replication, late syntheses, virion assembly, and release from the cell. Viral DNA Replication and virion assembly occur in the nuclei of infected Cells; the mRNA flow goes from The Nucleus to the Cytoplasm, and proteins synthesized in the cytoplasm are transported to the nuclei to the sites of virion assembly. Virions attach to specific cellular receptors on The cell membrane via fibers, enter the cell via endocytosis, and lose part of their capsid proteins in the process. Along microtubules, the virion is transported to the nuclear pores, where it loses most of its proteins while retaining histone-like internal proteins (V, VII), the terminal protein, and the low-molecular-weight (4,000) μ protein.

Early Transcription covers 5–18% of the genome length and is localized in the left 5' (E1A) and right 3' (E3) Regions of the genome. There are even finer distinctions in early syntheses: super-early (L1), pre-early (E1A), delayed-early (E1B–E4), and intermediate (IVa2, IX) genomic regions. The E1 (E1A) region is also of interest because it contains genes encoding the synthesis of transforming proteins (Fig. 43). Adenoviral mRNA synthesis is catalyzed by cellular RNA polymerase II, accompanied by cap structure formation, splicing, and polyadenylation. Cap structures for the E1A and E1B regions differ from each other.

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Fig. 43. Adenovirus genes and Gene products.

Viral mRNAs are indicated by arrows; L stands for late proteins, and E for early proteins. Relative values are plotted on the abscissa.

Early proteins E2A and E2B provide viral DNA replication, whereas proteins E1A and E1B mediate transformation. Tumor formation in hamsters is associated with the synthesis of early T Antigens, which, unlike the T antigens of Papovaviruses, are numerous. In addition to conventional mRNAs, infected cells synthesize low-molecular-weight VA (virus-associated) RNAs. This synthesis is catalyzed by cellular RNA polymerase III, and the RNA regulates the stages of late Translation.

Following cell infection by adenoviruses, DNA penetrates the nuclei and forms complexes with cellular Histones. Transcription of early genes takes place on this template. DNA replication is carried out under the "protection" of DNA-binding proteins and subsequently histones. Late gene transcription occurs on this structure. Newly synthesized DNA then forms complexes with core proteins and is packaged to form virions [Dery C. et al., 1985]. Only a few adenovirus genes have been studied so far. For instance, lytic and transforming activity is associated with the E1A gene, replication and transformation with the E1B gene, and cell immortalization with the E1 region.

Adenoviral DNA replication is accompanied by a 90% inhibition of host DNA Synthesis. Unlike host DNA replication via The formation of Okazaki fragments, adenoviral DNA synthesis occurs continuously from both of its ends. In this process, a 55,000-molecular-weight terminal protein covalently linked to the 5' ends of the strands serves as a primer for the initiation of viral DNA synthesis. Other Variants of the adenoviral DNA replication model have been proposed, taking into account the participation of additional factors of cellular and viral origin (Fig. 44).

Fig. 44. Protein primer model for the initiation of adenoviral DNA synthesis. Newly formed DNA strands (1) are covalently bound to the protein.

With the onset of DNA replication, early syntheses are shut off and replaced by late syntheses. Most polypeptides synthesized in the late stage are structural proteins or their precursors. However, some synthesized proteins (molecular weights 100,000, 50,000, and 39,000) are non-structural, although they participate in virion assembly. The latter occurs in the nuclei and is a multistep process. First, hexons are formed from polypeptide II trimers, along with pentons. A non-structural protein with a molecular weight of 100,000 participates in this process. Next, "light capsids" (600S, 1.315 g/ml) are formed, containing, in addition to genomes, proteins VI, VIII, IIIa, and proteins with molecular weights of 55,000 and 39,000. DNA enters the assembling capsids through one of the open vertices, and the capsids become denser (600S, 1.35 g/ml). Progeny virions mature while accumulating in the nuclei. In the process, cellular syntheses are completely blocked and the cells die.

Along with the productive infection described above, integrative processes may occur during cell infection. It is even suggested that replication includes rapidly transient stages of Integration of the viral genome with the cellular genome. As for the integrative type of interaction, it occurs predominantly upon infection of cells with highly oncogenic adenoviruses. Typically, not the entire adenovirus genome integrates, but only a part of it carrying oncogenes in the early transcription region. The early gene regions (E1A, E1B) serve as the starting point for replication and transformation. It is believed that the initiation of transformation and its maintenance are mediated by different genes, or rather their products [Hurwitz D., Chinnadurai G., 1985], with cellular genes (the pp60^c-^src protein) participating in this process. This holds true not only for adenoviruses, but also for other oncogenic Introduction/6.html">DNA-containing viruses, such as polyoma and papillomaviruses [Amini S., et al., 1986].

From an evolutionary standpoint, the oncogenic properties of adenoviruses and several other oncogenic DNA-containing viruses may seem pointless, especially since they often do not cause cell transformation in their natural hosts. This applies in particular to adenoviruses that persist for a long time in the Tonsils yet do not cause Cancer and sarcomas of the respiratory tract and the upper Esophagus. Some light is shed on this peculiar evolutionary pathway by studies of the early region (1B) of the highly oncogenic adenovirus type 5. This region, spanning 0–11.2 map units of the genome, contains two distinct transcriptional units, E1A and E1B, whose products are required for both lytic infection and transformation. Mutations in the E1B region affecting genes encoding antigens with a molecular weight of 19,000 or 21,000 are accompanied by the degradation of chromosomal DNA [White E. et al., 1984] or viral and cellular DNA [Pilder S. et al., 1984]. Among other characteristics of adenoviruses, two deserve special mention: the existence of defective adenovirus satellites and a specific type of interaction between adenoviruses and papovaviruses. The former is discussed in Chapter 22, whereas the latter is worth addressing here.

The interaction between adenoviruses and SV40 has a peculiar character. Adenoviruses multiply in certain cell types only in the presence of SV40, which acts as a helper in this case, since its removal (e.g., using immune serum) also halts adenovirus multiplication in monkey Kidney cells due to the blockage of the synthesis of certain structural proteins. In this case, the SV40 T antigen acts as a trans-activating factor that binds to adenoviral DNA more actively (than the adenovirus terminal protein), which is necessary to initiate its replication.

Adenoviruses form recombinants upon co-cultivation, and complementation can occur between mutants. Among other genetic features, of interest is their ability to form hybrids with SV40 that include the entire genome or a part of the papovavirus genome. Adenoviruses also support the reproduction of defective Parvoviruses—adenovirus satellites or adeno-associated viruses. As already mentioned, the adenovirus family is divided into two genera; viruses within a genus show serological relatedness, whereas serological links between genera are absent.

The genus Mastadenovirus comprises over 80 representatives infecting humans, monkeys, horses, cattle, sheep, goats, pigs, dogs, and mice. Although adenoviruses can multiply in Tissues of various animals, under natural conditions they infect one or a few closely related species.

Human adenoviruses are subdivided into 6 groups designated by Latin letters based on several properties, including oncogenic potentials. Viruses of serotypes 1–35 cause respiratory diseases, Conjunctivitis, pharyngoconjunctival fever, and tonsillitis, and they are easily cultivated in cell cultures; viruses of serovars 36–42 cause intestinal diseases and are non-cultivable. Highly Oncogenic Viruses (group A, serovars 12, 18, and 31; group C, serovars 2, 4, etc.) are oncogenic in newborn hamsters and cause cell culture transformation, but they are not oncogenic to humans, although they can persist in the tonsils for a long time. Oncogenic potentials are also possessed by certain adenoviruses infecting monkeys and other animals. In animals, adenoviruses cause respiratory lesions, keratoconjunctivitis, gastrointestinal disorders, and hepatitis in dogs. Many of them cause subclinical and asymptomatic infections, which, however, also applies to human adenoviruses.

The genus Aviadenovirus comprises 15 representatives infecting domestic (chickens, turkeys, ducks, geese) and wild (quail, pheasants) birds. These viruses cause respiratory lesions and other diseases, including fatal embryo infections.

This clearly defined group of adenoviruses has no analogs among other viral groups. Even if the presence of a terminal protein at the 5' end of DNA were considered an evolutionarily common feature, this trait is found in viruses from many diverse groups. In addition to RNA-containing Picornaviruses and plant disease viruses (sobemo-, como-, and neoviruses), which form a group with a uniform genome strategy, terminal proteins are possessed by many DNA-containing viruses, numerous viruses containing single- and double-stranded RNA, and even maize Mitochondrial DNA [Garcia P. et al., 1986]. Naturally, with such a diversity of genetic structures possessing terminal proteins, their presence can hardly be taken into account to assess the evolutionary relatedness of viruses, although some phages (Cp-1, φ29) possess inverted repeats at the ends of the DNA molecule in addition to terminal proteins. Moreover, there are strong reasons to believe that the replication scheme of adenoviruses and phages φ29 and Cp-1 shares many common features [Vartapetyan A. B., 1982; Garcia P. et al., 1986]. Incidentally, Tailed Phages should be considered phylogenetically ancient. Still, it remains unclear how adenoviruses emerged while restricting their ecological niches to warm-blooded animals, including humans.

Subsequent evolutionary pathways are understandable. Apparently, the evolution of most viruses was coupled with the evolution of their hosts, which explains the distinct delimitation of the two adenovirus genera, whose divergence led to the loss of immunological links between representatives of different genera. At the same time, the existence of serological relatedness among viruses within genera and a high degree of DNA homology not only within groups (70–95%) but also between viruses infecting different species (10–25%) indicate an evolutionary connection between the genera formed in the course of evolution. From this perspective, human adenoviruses can be viewed as descendants of adenoviruses of their preceding primates, and The Diversity of human adenoviruses reflects the evolutionary success of this viral group.

Nevertheless, much remains unclear, most notably the presence of phenotypically unexpressed oncogenic potentials in adenoviruses. The oncogenes of adenoviruses are likely of cellular origin, as evidenced by the discovery of homologous regions between mammalian cell and adenovirus genomes. However, these genes long ago became adenovirus genes necessary for their reproduction. This may well hold the answer to the question, since, as is known, cellular oncogenes are not "designed" for carcinogenesis but perform important regulatory Functions. Apparently, this was the reason for the incorporation of cellular genes into the adenovirus genome in the distant past when these viruses originated. From this viewpoint, it is very characteristic that the E1A gene of adenovirus serotypes 2 and 5, which yields 3 transcripts (18S, 12S, and 9S), provides both lytic and transforming activity. Similarly, gene 18 (58,000-MW) of adenovirus serotype 12 is an early gene whose product ensures the initiation of viral DNA replication as well as transformation. This "as well as" is apparently never realized during infection of its natural host by the virus, and the potential transforming activity of this gene, much like that of normal cellular oncogenes, is manifested only under special, artificially created conditions—during virus propagation in tissue culture or upon infection of newborn hamsters.

Here in the section on adenoviruses, it is also appropriate to mention two phage groups whose virion morphology resembles that of adenoviruses. The family Tectiviridae, comprising about 10 phages (the RRD group), has a double-stranded DNA genome with a molecular weight of 9×10⁶, which accounts for 14% of the virion mass. The virions contain 16–18 proteins. They feature an icosahedral structure with a diameter of 65 nm and club-shaped spikes at the vertices. The double capsid consists of a rigid outer shell and a flexible inner one. Replication has been poorly studied; these viruses infect many bacterial species. The family Corticoviridae includes 1–2 phages (the PM2 group) whose genome is a circular supercoiled DNA with a molecular weight of 6×106, accounting for 14% of the virion mass. The virions contain 4 proteins with molecular weights ranging from 5×10³ to 43×10³. Brush-like protrusions are present at the vertices. The virions also possess two capsids with a diameter of about 60 nm. Replication is poorly understood, and the viruses parasitize marine Pseudomonas species.

Any definitive insights into the ORIGIN AND EVOLUTION of these viruses are not yet possible.