Viruses
Many viruses consist only of:
Viruses infect all kinds of cells, those that infect bacteria are called bacteriophages or phages.
Three basic phage structures observed are:
tailless icosahedral phage - which consist of an icosahedral capsid
but no tail
icosahedral phage with tail
filamentous phage
The most common genome in phage is double-stranded linear DNA, but you also find ss DNA, ds RNA and ss RNA.
Genome size varies a lot between different types of phage- but in general- the more complex the genome, the more complex the phage.
The life cycles of phages fit into 2 categories:
Some phages can additionally undergo lysogenic cycle.
These phages are called temperate.
In lysogenic cycle phage DNA is usually inserted into the host chromosome
where it is silent (no phage progeny produced) and but is passed on from
cell generation to cell generation.
A phage that is integrated into the bacterial chromosome is called
a prophage.
Under appropriate conditions, DNA integration can be reversed and the
lytic cycle is initiated.
The LYTIC cycle is somewhat variable but the general features are as follows:
1) Adsorption
In this step, the phage attaches to the cell wall of the bacterial
cell. This is a highly specific interaction dependent upon the binding
of the phage to specific receptors on the cell surface.
2) Penetration
In this step, the phage genome passes through the bacterial cell wall
and enters the cytoplasm. This occurs within seconds of adsorption and
in the vast majority of cases, the capsid and additional structural elements
remain outside of the bacterial cell. Often and enzyme such as lysozyme,
is used to weaken the cell wall so that the genome can pass through.
3) Conversion of bacterium into a phage-producing cell
This step involves the shutdown of bacterial DNA and RNA synthesis
which may even extend to DNA fragmentation and degradation. Nucleotides
can then be used during viral DNA replication!.
4) Biosynthesis
In this step, phage nucleic acids and proteins are synthesized.
Phage DNA is replicated
Phage DNA is transcribed into phage mRNA
Phage mRNA is translated into phage proteins
Transciption is regulated, i.e. not all bacteriophage DNA is transcribed
at the same time. Phage mRNA and phage proteins are synthesized as needed.
Early mRNAs:
mRNAs which encode the enzymes required for the take over of a bacterium
mRNAs which encode the proteins required for DNA replication
mRNAs which encode proteins that are required for the transcription
of Late mRNAs.
Late mRNAs
mRNAs which encode phage structural proteins (capsid proteins, tail
sheath proteins, etc.)
mRNAs which encode proteins needed for packaging phage nucleic acid
into newly formed capsids.
mRNAs which encode proteins required for lysis of the bacterial cell.
5) Morphogenesis
In this step, the progeny phage are assembled. Structural proteins
are assembled to form capsid, tail sheath, tail fibers, etc. In addition,
the phage genome is packaged inside the newly-formed capsids.
6) Lysis
In this step, the progeny phage are released then the bacterial cell
undergoes lysis. Lysis is stimulated by the release of enzymes such as
lysozyme or endolysin. Obviously these two enzymes are synthesized
very late in the cycle of infection.
The sequential events of the lytic cycle occur over a 20-60 second period depending upon the individual type of phage involved. This time period is termed: Burst Time
Certain bacteriophage also have an optional lysogenic cycle. If environmental conditions are poor, the phage genome can survive by integrating into the bacterial chromosome.
Example for a temperate phage: bacteriophage lambda (lambda is
a greek letter). (Fig. 5.37)
Bacteriophage lambda consists of:
After infection, the linear lambda DNA is converted to a circular, supercoiled form and transcription of lambda genes starts. At this point, decision is made on which life cycle to follow (see Fig. 5.37).
What decides? Seems to be the concentration of an activator protein
transcribed from lambda DNA this activator is designated cII:
high concentration of cII ---- lysogenic cycle favored
low concentration of cII ---- lytic cycle favored
But: concentration of activator not dependent on lambda but
on host cell: concentration determined by amount of proteases present
in the host cell which in turn depends on growth conditions:
favorable growth conditions ---- high amount of proteases ---- high degree of activator degradation ---- low levels of cII ----- lytic cycle
poor growth conditions ---- low amount of proteases ---- low degree of activator degradation ---- high levels of activator ----- lysogenic cycle
Therefore, phage progeny only produced when cell has many resources.
When infecting resource-depleted bacterium, lambda can "hide" in cell
Lysogenic Cycle:
Effect of prophage:
Bacterium is now immune to infection by another lambda phage,
because lambda repressor continuously produced ----- new phage DNA can
be injected into cell and is circularized but is not transcribed or replicated.
But prophage can be excised, because lambda cleverly uses host
response system to potentially lethal situations:
TRANSDUCING PHAGE
Term transduction refers to the packaging of bacterial DNA (chromosomal DNA) into the phage capsid. This occurs with only a small % of phage.
In specialized transduction
During induction, a prophage is excised from the bacterial chromosome
and carries along with it some of the chromosomal DNA. In other words,
there is an imprecise excision of the prophage DNA. The chromosomal DNA
probably does not represent entire genes, but when this phage infects a
bacterial cell, homologous recombination is a likely possibility. [very
similar to formation of F' bacterium]
In generalized transduction
Bacterial DNA fragments are randomly (and accidentally) packaged into
the capsid along with the phage genome. This occurs during the lytic cycle
and is especially common in situations in which the host cell DNA is fragmented
during conversion phase.