13.1: Inheritance of Genes during Virus Infection Cycles

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

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

• DNA or RNA
• ss or ds
• single DNA or RNA molecule
• segmented genomes
• their genes are carried by a number of different RNA molecules

• 3
• 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

more complex capsid structures of these bacteriophages

a dependence on a greater number of bacteriophage-encoded enzymes during the infection cycle

• 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

Add T4 bacteriophages to culture of E. coli

3 minutes= attachment

Measure at forty 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

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

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

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

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.

early and late genes

• genome replication
• synthesis of capsid proteins
• synthesis of lysozyme
• the very end of the infection cycle

• exerts no control over transcription of its genes
• 11
• transcribed by the host RNA polymerase
• Genome replication and capsid synthesis
• lysozyme synthesis
• mRNA for this enzyme is translated slowly

early (products are needed during the early stages of infection) and late genes (remain inactive until toward the end of the cycle)

very early

• these groups of genes are expressed int eh correct order
• cascade system
• switches on transcription of the next set of genes

E. coli promoter sequences present on the phage genome

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.

• new sigma subunit, sigma55
• replaces the host’s sigma70 version
• transcribes a third set of bacteriophage genes

• lytic infection cycle
• integrated
• immediately after entry
• prophage

site-specific mechanism

integrates at the same position within the E. coli DNA molecule

• 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

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

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.

the dual role of the cI repressor

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

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.

early-gene expression is blocked and lysogeny follows

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

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

Inactivation of the cI repressor, which typically maintains lysogeny because it binds to the operators adjacent to pL and pR

• levels of active cI repressor decline below a certain point
• spontaneous induction
• general protective mechanism
• SOS response

• 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

• transcription of the cI gene is no longer stimulated
• synthesis of more cI repressor
• induction of the prophage

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

• synthesized
• assembled and released from the cell
• cell bursting

• passed on to daughter bacteria during cell division
• partly lytic and partly lysogenic