Transcript Freeman 1e: How we got there
CHAPTER 16 Viral Diversity
Viruses of Prokaryotes RNA Bacteriophages
• A variety of RNA
viruses
that infect bacteria are known. The small RNA genome of these bacterial viruses is translated directly and encodes only a few proteins.
•
Figure 16.2a
shows the genetic map of RNA
bacteriophage
MS2, and
Figure 16.2b
shows the flow of events of MS2 multiplication.
• The small genome encodes only four proteins .
• These are the maturation protein (present in the mature virus particle as a single copy), coat protein , lysis protein (involved in the lysis process that results in release of mature virus particles), and a subunit of
RNA replicase
, the enzyme that brings about replication of the viral RNA.
Icosahedral Single-Stranded DNA Bacteriophages
• M13 and X174 are ssDNA viruses - ~25 nm •The single-stranded DNA genome (5356nt)of the virus X174 is so small that
overlapping genes
are required to encode all its essential proteins.
This virus provided the first example of overlapping genes.
• Because cellular DNA always replicates in the double-stranded configuration, the replication process of the single-stranded genome of X174 is of interest .
• On infection, the plus-sense viral DNA becomes separated from the protein coat. Entrance into the cell is accompanied by the conversion of this single stranded DNA into a double-stranded molecule called the
replicative form
(RF).
•
Figure 16.3a
shows the genetic map of phage X174, and
Figure 16.3b
shows the flow of events during X174 replication.
A mRNA is read twice by ribosomes, once for A and second for A *
• The production of progeny viral DNA involves
rolling circle replication
.
Figure 16.4
mechanism in phage X174.
shows the
A cleaves the plus strand of the RF
Filamentous Single-Stranded DNA Bacteriophages
• Some single-stranded DNA viruses, such as M13, have filamentous
virions
(
Figure 16.5
) related to f1 and fd phages .
•These viruses are very useful tools for DNA sequencing and genetic engineering. They are released without actually killing the host.
•M13 is only 6 nm in diameter but 860 nm long.
•Filamentous phages are released without killing the host cell.
•All phages with protein A (including X174) do not kill host cells.
Release of phages
Double-Stranded DNA Bacteriophages: T7
• The bacteriophage T7 double-stranded DNA genome always enters the host cell in the same orientation.
Figure 16.6
shows the genetic map for T7.
• The late genes in T7 are transcribed by a virus-encoded RNA polymerase . •The replication strategy for the T7 genome employs T7 DNA polymerase and involves terminal repeats and the formation of
concatemers
(
Figure 16.7
).
Formation of concatamers by joining DNA at the unreplicated terminal ends
Production of mature viral DNA
Mu: A Double-Stranded Transposable DNA Bacteriophage
• Bacteriophage Mu is a temperate virus that is also a transposable element . In either the lytic or lysogenic pathway, its genome is integrated into the host chromosome by the activity of a
transposase
.
• Even in the lytic pathway, its genome is replicated as part of a larger DNA molecule. The genome is packaged into the virion in such a way that there are short sequences of host DNA at either end.
•
Figure 16.9
illustrates replication of bacteriophage Mu. Genome – 39 kb (37.2 kb viral DNA and 1.8 kb host DNA) 50-150 bp 1-2 kb Lambda. T4, T7, and Mu have linear dsDNA
Viruses of Eukaryotes Plant Viruses
• Most plant viruses have
positive-strand
RNA genomes. One example is tobacco mosaic virus (TMV), the first virus discovered (
Figure 16.11
).
• The genomes of these viruses can move within the plant through intercellular connections that span the cell walls.
• Other types of plant viruses are also known, including the
Chlorella
viruses , which have very large double-stranded DNA genomes .
Positive-Strand RNA Viruses of Animals: Poliovirus and Coronaviruses
• In small RNA viruses such as poliovirus, the viral RNA is translated directly , causing the production of a long
polyprotein
that is broken down by enzymes into the many small proteins necessary for nucleic acid multiplication and virus assembly (
Figure 16.13
).
Poliovirus
RNA linked 22 a. a. protein serve as a primer
• Coronavirus is a large single-stranded RNA virus that resembles poliovirus in some but not all of its replication features (
Figure 16.14
).
Coronavirus – Flow of information
Negative-Strand RNA Viruses of Animals: Rabies, Influenza, and Related Viruses
• In
negative-strand viruses
, the virus RNA is not the mRNA but is copied into mRNA by an enzyme present in the virion. •
Figure 16.16
illustrates the flow of events during multiplication of a negative-strand RNA virus.
•
Vesicular stomatitis virus (VSV)
• Translation of viral mRNAs leads to the synthesis of viral coat proteins. •Assembly of an
enveloped
virus is considerably more complex than assembly of a naked virion.
• Two kinds of coat proteins are formed,
nucleocapsid
proteins and envelope proteins. The nucleocapsid is formed first by association of the nucleocapsid protein molecules around the viral RNA.
• Important negative-strand viruses include rabies virus and influenza virus (
Figure 6.17
).
influenza virus
Influenza virus
Double-Stranded RNA Viruses: Reoviruses
• Reoviruses contain segmented double stranded RNA genomes. •Like negative-strand RNA viruses, reoviruses contain an RNA-dependent RNA polymerase within the virion.
Replication of Double-Stranded DNA Viruses of Animals
• Most double-stranded DNA animal viruses, such as SV40 , replicate in the nucleus. SV40 has a tiny genome and employs the strategy of overlapping genes to boost its genetic-coding potential. Some of these viruses cause cancer.
•
Figure 16.20
shows the genetic map of polyomavirus SV40.
5.2 kb
•
Figure 16.21
shows the general scheme of molecular events involved in cell transformation by a polyomavirus such as SV40.
• Herpesviruses cause a variety of disease syndromes and can maintain themselves in a latent state in the host indefinitely , initiating viral replication periodically.
Double-Stranded DNA Viruses: Herpesviruses
• Herpesviruses are large, double-stranded DNA viruses. The viral DNA circularizes and is replicated by a rolling circle mechanism .
•
Figure 16.22
illustrates the flow of events in multiplication of herpes simplex virus.
Double-Stranded DNA Viruses: Pox Viruses
• The pox viruses, unlike the other DNA viruses discussed so far, are very large viruses that replicate entirely in the cytoplasm.
These viruses are responsible for several human diseases, but a vaccination campaign has eradicated the smallpox virus in the wild.
Double-Stranded DNA Viruses: Adenoviruses
• Different double-stranded DNA animal viruses have different genome replication strategies.
• The strategy of the adenoviruses involves protein primers and a mode of replication that avoids the synthesis of a lagging strand and occurs within the nucleus (
Figure 16.24
).
Viruses Using Reverse Transcriptase: Retroviruses and Hepadnavirus
• The
retroviruses
contain RNA genomes and use
reverse transcriptase
to make a DNA copy during their life cycle (
Figure 16.25
).
Lys3
•
Figure 16.26
illustrates translation of retrovirus mRNA and processing of the proteins.
• The
hepadnaviruses
contain DNA genomes and use reverse transcriptase to make genomic DNA from an RNA copy .
Figure 16.27
shows the genome of hepatitis B, a hepadnavirus.
Hepadnaviruses (Liver infecting) – Small, irregular, rod shaped
Hepatitis B virus – serious bloodborne pathogen 3-4 kb DNA genome is partially ds Replicated through RNA intermediate
•These viruses have complex patterns of gene expression.