13.1: Inheritance of Genes during Virus Infection Cycles
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are common and have been viewed as possible controls or treatments for bacterial infections
have two components: protein forms the coat/ capsid) and nucleic acid
Three types of bacteriophages
Icosahedral: individual protein subunits (protomers) are arranged into a 20-faced geometric structure that surrounds the nucleic acid Ex: coli
Filamentous capsids: protomers are arranged in a helix, producing a rod-shaped structure
Head-and-tail bacteriophages: combine the features of the other two types. Their capsid is made up of an icosahedral head, containing the nucleic acid, and a filamentous tail, which facilitates entry of the nucleic acid into the host cell
Bacteriophages can have __ or __, which can be __ or __
With most types of bacteriophage there is a __ that comprises the entire genome. A few RNA bacteriophages have __, meaning that __.
- DNA or RNA
- ss or ds
- single DNA or RNA molecule
- segmented genomes
- their genes are carried by a number of different RNA molecules
DNA sequencing was done on viruses. They can have __ to __ genes. The smaller bacteriophage genomes contain few genes, whch can be organized in a very complex manner, such as by __, which __, but __ because __. This is not uncommon in viruses
- over 100
- having overlapping genes
- share nucleotide sequences
- code for different gene products
- the transcripts are translated from different start positions and in different reading frames
Larger bacteriophage genomes contain more genes, reflecting the __ and __
more complex capsid structures of these bacteriophages
a dependence on a greater number of bacteriophage-encoded enzymes during the infection cycle
Bacteriophages are classified into two groups—__ and __, the difference being that __, whereas a__
Examples of each: lytic (virulent) T4 and the lysogenic (temperate) lambda
- lytic and lysogenic
- lytic kills its host bacterium very soon after the initial infection, usually within 30 minutes
- lysogenic bacteriophage can remain quiescent within its host for a substantial period of time
Tie line of T4 infection cycle
Add T4 bacteriophages to culture of E. coli
3 minutes= attachment
Measure at forty minutes
What happens during the first 22 minutes?
1. There is no change in the number of infected cells during the first 22 minutes of infection. This is the latent period and is the time needed for bacteriophages to reproduce within their hosts
What happens after 22 minutes?
2. After 22 minutes the number of infected cells starts to increase, showing that lysis of the original hosts is occurring, and that the new bacteriophages that have been produced are now infecting other cells in the culture. This infection cycle is the one-step growth curve
c. Molecular events occurring during the T4 infection cycle
1) Attachment of bacteriophage to receptor protein on outside of bacterium (receptors vary)
2) Injection of DNA genome into cell
3) Latent period begins, which invovles frenzied activity directed at synthesis of new bacteriophage particles
Example of attachment of phage to protein
1. Ex: for T4, the receptor is OmpC, which is a type of outer-membrane protein called a porin, which forms a channel through the membrane and facilitates the uptake of nutrients
Explain after entry of phage DNA
Immediately after entry of the bacteriophage DNA, the synthesis of host DNA, RNA, and protein stops and transcription of the phage genome begins.
Within 5 minutes the bacterial DNA molecule has been broken down and the resulting nucleotides are being utilized in replication of the T4 genome.
After 12 minutes new phage capsid proteins start to appear and the first complete phage particles assemble
At the end of the latent period, the cell bursts and the new phages are released.
The lytic infection cycle is regulated by expression of __ and __
early and late genes
With most bacteriophages, __ precedes __. Similarly, __, the enzyme that causes the bacterium to burst, must be delayed until __. Individual bacteriophage genes must therefore be expressed at different times in order for the infection cycle to proceed correctly
- genome replication
- synthesis of capsid proteins
- synthesis of lysozyme
- the very end of the infection cycle
Example via φX174, a phage that __. All __genes are __ as soon as the bacteriophage DNA enters the cell
__ and __ occur more or less at the same time, but __ is delayed because the__
- exerts no control over transcription of its genes
- transcribed by the host RNA polymerase
- Genome replication and capsid synthesis
- lysozyme synthesis
- mRNA for this enzyme is translated slowly
With most other bacteriophages, there are distinct phases of gene expression. Two groups of genes are usually recognized—__ and __
There may be other divisions within these groups, some phages having “__” genes
early (products are needed during the early stages of infection) and late genes (remain inactive until toward the end of the cycle)
A number of strategies are employed by pahges to ensure that __, utilizing a __—the appearance in the cell of the translation products from one set of genes does what?
- these groups of genes are expressed int eh correct order
- cascade system
- switches on transcription of the next set of genes
Wth T4, the very first genes to be expressed are transcribed by the__
E. coli promoter sequences present on the phage genome
Explain T4 transcription products
Very early gene products: proteins that modify the sigma subunit of the host RNA polymerase so it no longer recognizes E. coli promoters, thereby switching off host gene expression. Instead, the RNA polymerase now specifically transcribes a second set of bacteriophage genes.
One of these genes specifies a __, which does what, so the RNA polymerase now does what?
The individual groups of genes are therefore expressed in the correct order, the products of one set switching on expression of the next set.
- new sigma subunit, sigma55
- replaces the host’s sigma70 version
- transcribes a third set of bacteriophage genes
Bacteriophage lambda, like most temperate bacteriophages, can follow a __ but as an alternative.
This genome gets __into the host DNA
Integration occurs __; and a __is formed.
- lytic infection cycle
- immediately after entry
Integration occurs by a __ involving identical 15-bp sequences present in the lambda and E. coli genomes, meaning that the lambda genome always __.
integrates at the same position within the E. coli DNA molecule
The integrated prophage can be __, being replicated with the bacterial genome and inherited. The switch to the lytic mode of infection occurs if __, each appearing to be linked to __ and thus signaling the imminent death of the host by natural causes
- retained in the host DNA molecule for many cell generations
- the prophage is induced by any one of several chemical or physical stimuli
- DNA damage
How is lysogeny maintained?
First step in the lytic infection cycle is expression of teh early lambda genes, which are transcribed from two promoters, pL and pR, located on either side of a regulatory gene called cI
During lysogeny, pL and pR are switched off because the cI gene product, which is a repressor protein, is bound to operators adjacent to these promoters--> early genes are not expressed and the bacteriophage cannot enter the lytic cycle
Why is lysogeny maintained for numerous cell divisions?
becuase cI gene is continuously expressed so that the amount of cI repressor presentis enough to keep pL and pR switched off.
This continued expression of cI occurs because the cI repressor not only blocks transcription from pL and pR, but also stimulates transcription from pM, the promoter for the cI gene.
What is key to lysogeny?
the dual role of the cI repressor
How does the bacteriophage “decide” whether to follow the lytic or the lysogenic cycle?
This depends on the outcome of a race between the cI and cro proteins.
When a lambda DNA molecule enters an E. coli cell, the host's RNA polymerase enzymes attach to the various promoters on the molecule and start transcribing the lambda genes
--> Once the cI gene is expressed, the cI repressor blocks expression of the early genes, preventing entry into the lytic cycle and enabling lysogeny to be established
Why is lysogeny not always the outcome of a lambda infection?
Because a second gene, cro, also codes for a repressor, one that prevents transcription of cI.
Both the cI and cro genes are expressed immediately after the lambda DNA molecule enters the cell.
What is the outcoe if the cI repressor is synthesized more qucikly?
early-gene expression is blocked and lysogeny follows
What is the outcoe if the cro repressor is synthesized more qucikly?
However, if the Cro repressor wins the race it blocks expression of the cI gene before enough cI repressor has been synthesized to switch the early genes off
As a result, the bacteriophage enters the lytic infection cycle
The decision between lysis and lysogeny therefore depends on?
which of the two gene products, cI and Cro, accumulate teh quickest.
The decision is influenced by the products of other lambda genes, which are able to assess the physiological state of the host cell, and hence ensure that the appropariate choice is made between lysogeny and immediate lysis
How is lysogeny ended?
Inactivation of the cI repressor, which typically maintains lysogeny because it binds to the operators adjacent to pL and pR
The prophage will therefore be induced if the __.
This may happen by chance--> __
May occur by physical or chemical stimuli, which activate a __ in E. coli, the __
- levels of active cI repressor decline below a certain point
- spontaneous induction
- general protective mechanism
- SOS response
Part of the SOS response is __
What does RecA do? RecA__
- expression of an E. coli gene, recA, coding for the RecA protein
- inactivates the cI repressor by cutting it in half, which switches on the expression of the early genes, enabling the bacteriophage to enter the lytic cycle
Inactivation of the cI repressor also means that __, avoiding the possibility of lysogeny being reestablished through the __.
Inactivation of the cI repressor therefore leads to __
- transcription of the cI gene is no longer stimulated
- synthesis of more cI repressor
- induction of the prophage
True or False:
Although lysis and lysogeny are the two most typical bacteriophage life cycles, they are not the only ones
True: One or two other bacteriophages display unusual infection cycles that are neither truly lytic nor truly lysogenic
This genome is replicated by synthesis of the complementary strand, producing a double-stranded, circular DNA molecule, which undergoes further replication until there are 100+ copies in the cell
At this stage, the infection cycle takes on characteristics of both lytic and lysogenic bacteriophages
As with lytic bacteriophages, M13 coat proteins are __, and new bacteriophage particles are __.
However, as with lysogenic bacteriophages, __ does not occur and the infected bacteria continue to grow and divide
- assembled and released from the cell
- cell bursting
Copies of the bacteriophage genome are __, and M13 assembly and release continues. The M13 infection cycle is therefore __.
- passed on to daughter bacteria during cell division
- partly lytic and partly lysogenic
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