VIRUS PROPERTIES Infectious – must be transmissible horizontally Intracellular – require living cells RNA or DNA genome, not both* Most all have protein coat* May of may not have lipid envelope May have broad or narrow host range Replication involves eclipse (breaking apart of virus particles) and reassembly Use host factors for to complete replication cycle

SOME CONSEQUENCES AND EFFECTS OF VIRUS INFECTION Like other life forms, viruses promote the propagation of their own kind Like other life forms, viruses evolve in response to selection pressure Viruses are major factors in promoting the evolution of higher organisms Viruses help control populations of their hosts, including humans

Virus-like agents classified and studied with viruses Viroids –No coat protein, no coding capacity Prions –No nucleic acid (?) Retrotransposons –No infectivity (?)

Host properties influence the virus types found in that host group Vertebrates have broad range of viruses Plants have mostly small RNA viruses Fungi have mostly dsRNA viruses Single-celled organisms have mostly large dsDNA viruses

Viruses are submicroscopic intracellular pathogens

Flint et al., 2004 Principles of Virology Fig. 1.8 Sizes of microscopic and submicroscopic biological entities and their ability to be examined using various technologies

Viruses may be simple or complex Genome sizes kb; average genome sizes vary with host organism types Isometric particle sizes vary from ~24 nm to ~400 nm diameter May have single-stranded (ss) or double- stranded (ds) RNA or DNA genome If ssRNA, may be + or – sense May have one or many proteins in particles May or may not have lipid envelope

Types of viral genomes double-stranded (ds) DNA –Rarely segmented –Often large single-stranded (ss) DNA –Rarely large –Less common than dsDNA ssRNA, negative sense –Often found in viruses with broad host ranges ssRNA, positive sense –Most common overall dsRNA –Often segmented –Particle structure often critical

Composition of viruses infecting different hosts No “rules” about virus families that may or may not be present in a given kingdom Some types of viruses are found more commonly in some kingdoms than in others –Many plant viruses contain ssRNA genomes –Many fungal viruses contain dsRNA genomes –Many bacterial viruses contain dsDNA genomes Host properties determine the types of viruses that tend to be found in members of a biological “kingdom”

Virus types by nucleic acid composition DNA RNA ss ds ssds env naked env naked Families Species Host type Vertebrate Invertebrate Plant Fungus Bacteria

Overview of Virus Properties Animal –RNA – 5-30 kb –DNA: kb –Many enveloped –Range of complexity –Range of morphologies –Some divided genomes Prokaryote –RNA – 5-8 kb – DNA – kb –Few enveloped –Range of complexity –Range of morphologies –Few divided genomes True Fungi – RNA – kb –DNA – none –Enveloped ones have no capsid –Little genome complexity –Little morphological complexity –Some divided genomes Plant – RNA – kb –DNA – 3-10 kb –Few enveloped –Little genome complexity –Little morphological complexity –Many divided genomes Lower eukaryote –RNA – 5-10 kb – DNA – kb –Internal envelope –Range of complexity –Range of morphologies –No divided genomes

Tobacco mosaic virus – a typical small RNA virus 18X300 nm Single 6400 nt RNA 2130 copies of single 17 kDa coat protein 3 essential genes Simple regulatory elements

Poxvirus – a typical large dsDNA virus 120X200 nm Single 180 kb DNA Complex coat made up of numerous proteins >100 essential genes Complex regulatory elements

Virus at the edge: Mimivirus Mimivirus infects Acantamoeba polyphaga 400 nm particle, 1.2 megabase genome, 1262 putative ORFs represent the largest virus identified yet Many genes for normal cellular functions –central translation functions Aminoacyl tRNA synthetases Peptide release factor 1 Translation elongation factor EF-TU –DNA repair enzymes –Many polysaccharide synthesis enzymes Lineage suggests connection with eukaryotes, not prokaryotes Raoult et al., Science Express 10/14/04

Phylogenetic position of Mimivirus Compared to other similar DNA viruses Compared to other prokaryotic and eukaryotic life forms Raoult et al., Science Express 10/14/04

Mimiviruses (arrowed) can bee seen inside their amoeba host, Acanthamoeba polyphaga using a light microscope Raoult et al., Science Express 10/14/04

By transmission electron microscopy, isolated Mimivirus can be seen as a large icosahedral virus with fibrilar protrusions Mimivirus (green) seen by fluorescence microscopy in amoeba Mimivirus in ultrathin section in amoeba Raoult et al., Science Express 10/14/04

At 1.2 megabases (1.2X10 6 nucleotides), Mimivirus has the largest genome of any known virus, larger than many bacteria

Simple virus replication cycle 1.Virus enters host cell 2. RNA released; translates 3. Replication in cytoplasm 4. New virus assembled 5. New virus released from host cell

From 7 th Report of the ICTV (Academic Press, 2000)

From 7 th Report of the ICTV (Academic Press, 2000)

From 7 th Report of the ICTV (Academic Press, 2000)

From 7 th Report of the ICTV (Academic Press, 2000)

From 7 th Report of the ICTV (Academic Press, 2000)

From 7 th Report of the ICTV (Academic Press, 2000)

From 7 th Report of the ICTV (Academic Press, 2000)

From 7 th Report of the ICTV (Academic Press, 2000)

From 7 th Report of the ICTV (Academic Press, 2000)

From 7 th Report of the ICTV (Academic Press, 2000)