Small Single Stranded DNA Viruses of Bacteria

Bacteriophage FX174 is a small simple spherical virus that contains 5,386 bases of circular single-stranded DNA. At each of the 12 vertices of the virion is a spike made of bacteriophage FX174 A small spherical virus that contains circular single-stranded DNA and infects Escherichia coli RNA replicase Special RNA polymerase used by RNA viruses to replicate their RNA genomes

FIGURE 17.10 Filamentous and Spherical Virus Structures

Filamentous viruses are actually built form helical arrays of protein. The proteins of tobacco mosaic virus coat the helical RNA molecule to form a cylinder. Spherical viruses, such as adenovirus, are actually icosahedral. They have a 20-sided form that may or may not have protein spikes sticking out from the protein coat at the vertexes. These protruding knobs carry the proteins that recognize virus receptors on the host cell surface.

Protein

Protein

HELICAL (part of tobacco mosaic virus)

ICOSAHEDRAL (adenovirus)

FIGURE 17.11 Rhabdovirus is an Enveloped Virus

Rhabdoviruses are examples of enveloped RNA viruses. The group includes vesicular stomatitis, rabies and related viruses. The envelope is generated by budding of the virus from the cytoplasmic membrane of the infected host cell, as shown. (I) The viral ribonucleoprotein (RNP) associates with the inside of the membrane via M protein. (II) RNP condensation begins and bud sites form. Budding occurs preferentially where the viral glycoprotein (G) has clustered. (III) Assembly and extrusion of the virion. (IV) Cellular proteins of unknown identity (?) complete the budding and promote release of virions (V) via membrane fission. The inset shows a virion in cross-section with its central M protein scaffold and glycoprotein spikes protruding from the viral envelope. From Jayakar et al, Rhabdovirus assembly and budding. Virus Research (2004) 106:117.

Complex Bacterial Viruses with Double Stranded DNA 465

FIGURE 17.12 Bacteriophage FX174 has Overlapping Genes

Since many viral genomes are very small, the same region of the genome sometimes encodes two different genes. In the FX174 genome, the gene for E protein is entirely embedded within the gene for D protein. The two gene sequences are not identical, even where they overlap, since D uses a different reading frame than E.

ssDNA Gene C

Genes D and E share same DNA sequence

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