Medium sized, non-enveloped viruses with linear, non-segmented double-stranded DNA genome. Adenoviridae

Description of Adenoviridae adapted from ICTVdb Virion Properties Morphology Virions consist of a capsid, fibers, a core, and associated protein(s). Virus capsid is not enveloped, round with icosahedral symmetry. The isometric capsid has a diameter of 80-110 nm. The capsid shells of virions are composed of a single layer. Capsids appear hexagonal in outline. The capsid surface structure reveals a regular pattern with distinctive features. The capsomer arrangement is clearly visible. The capsid consists of 252 capsomers; 12 of which are pentons; 240 are hexons; each capsomer measures 8-10 nm in diameter. Surface projections are distinct, one or two filaments; protruding from the 12 vertices. Hexons; consist of a hexagonal base with a central cavity. Penton bases are tightly associated with one or two fibers; fiber proteins interact to form a shaft of characteristic length with a distal knob. The length of fibers is between 9-77.5 nm. The core consists of a nucleoprotein complex. Physicochemical and Physical Properties The molecular mass (Mr) of virions is 150-180 x 106. Virions have a buoyant density in CsCl of 1.32-1.35 g cm-3. The thermal inactivation point (TIP) is at 56°C. Under in vitro conditions virions are stable when stored at -20°C; stable in acid environment of pH 5-6. Virions are not sensitive to treatment with lipid solvents. Nucleic Acid The genome is not segmented and contains a single molecule of linear double-stranded DNA. The complete genome is 35800-36200 nucleotides long. The genome has a guanine + cytosine content of 48-61% for Mastadenovirus and 54-55% for Aviadenovirus 55 %. The genome has terminally redundant sequences. The terminally redundant sequences have inverted terminal repetitions (ITR) (103bp for human adenovirus 2, ITR's between 50-200 bp found in all viruses so far analyzed). The genome has a virus coded terminal protein which is covalently linked to the 5'-end of each DNA strand. Replication is divided into EARLY and LATE phases, the latter defined as beginning with the onset of DNA replication. Attachment to cells is rather slow, taking several hours to reach a maximum. Genome Organization and Replication Virions attach fibers to enter host cells by endocytosis. The process of intracellular uncoating of virions is understood. Virus uncoating occurs in the cytoplasm; the viral core is delivered to the cell nucleus; the site of mRNA transcription or DNA replication. By itself, genomic nucleic acid is infectious. Infection and Replication: Host cell DNA synthesis is shut-off; early in the replication cycle. Host cell RNA and protein synthesis is shut-off; later in the replication cycle. Transcription: Virus transcription is temporally regulated; 2 classes of genes recognized; they are termed EIA and E1B; E2A and E2B; E3 and E4 early and late (L1, L2 and L3; L4 and L5). The viral genome is transcribed from both DNA strands by host cell enzymes. During the early stage, the viral genome is transcribed by eukaryotic nuclear RNA polymerase II; late stage, the viral genome is transcribed by eukaryotic nuclear RNA polymerase III. Cellular RNA polymerase III; encode products which facilitate translation; of late mRNAs. Viral genes are expressed from an early promoter (that is E1-E4, two intermediate), or a later promoter (L). Viral mRNA(s) is/are transcribed from four early E1-E4, two intermediate promoters and one major late (L) promoter 5 promoter(s, in an ambisense coding arrangement; all. Primary transcripts are capped; polyadenylated. Viral mRNA families are produced by complex splicing patterns. Coding Strategy of Segment 1: sequence encodes non-structural proteins. Sequence encodes protease. Translation: Structural proteins, or non-structural proteins. Synthesized in the nucleus. Translation of structural proteins occurs during the early transcription phase of replication (TP), or intermediate transcription phase of replication (IX and IVa2), or late transcription phase of replication (II-VIII, X). Translation of non-structural proteins occur in the early stage of replication (DBP DNA pol and others), or late stage of replication (maturation 52/55kDA, mu, 33kDa p protein and 100 kDa). Structural proteins, or non-structural proteins (many). Are modified by post-translational processes. Including proteolytic cleavage, or phosphorylation, or glycosylation. The genome replicates in the nucleus. Replication does not involve a reverse transcription step. Genome replication occurs by a strand-displacement. Genome replication uses a protein priming mechanism (terminal protein) (together with a), or virus-coded DNA polymerase (and), or DNA binding protein (in concert with), or cellular factors. Virions may provide helper functions to dependent virus during replication. Virion acts as helper for a satellite virus. Replication cycle The precursor of capsid protein is found in the infected cell nucleus. Viral proteins accumulate in the nucleus. Virions accumulate in the cell nucleus. Assembly and Egress: Viruses assemble in the nucleus (sometimes in paracrystalline arrays along with similar arrays of virus structural proteins). Maturation: Virions mature after proteolysis of some structural proteins by the virus-coded protease. Release: Virus is released from host cell. Virus is released from host cell by disintegration.