Bacteriophage -Lambda

Lambda Phage is a temperate phage infect E.coli cells.

Structure of the virus particle consist of Head( icosahedral in shape ) and Long flexible tail.

Head proteins encloses the 48.5kbp linear double stranded DNA molecule.

Binds to specific receptors on the outer membrane of E.coli

Injects its linear genome through the phage tail into the cell.

Once the linear double stranded DNA entered in to the host cell, it can either integrate into host chromosomal DNA and becomes a noninfectious prophage , or dont integrate but replicate their DNA molecule and cause lysis of the host cell.

Linear and Circular forms of lambda DNA Both the ends of linear ds DNA molecule are flanked by single stranded DNA of 12 nucleotides . These single stranded DNA molecules are complementary in sequence - referred as sticky ends or cohesive ends are also called cos sites

cos site have two distinct roles during infection cycle

Allow the DNA molecule to circularize once entered into the host cell. ( This is necessary for insertion into bacterial genome)

2. After excision of prophage, large numbers of new lambda DNA molecules are produced by rolling circle replication- results in catenane formation, which consist of series of linear lambda genomes joined together at cos sites.

Cos sites now act as recognition sequences for endonuclease - which cleaves catenane at cos sites. - This results in the formation of individual lambda genomes.

Endonuclease is a gene A product of Lambda DNA

Which creates single stranded sticky ends in lambda DNA

also involved in packaging of Lambda genome into the phage head in combination with other proteins.

Gene Organization in the Lambda DNA Genetic map of lambda phage shows that the functionally related genes are clustered together on the genome

Except gene N and Q Map shows that genes coding for tail and head proteins are grouped together on left

Central region responsible for integration of lambda DNA in to host genome Most of the genes in this region are non essential for phage growth (Hence these genes are deleted or replaced without impairing the infectious growth cycle)

Life cycle of Bacteriophage Lambda (Lytic and Lysogenic)

Promoters and control circuitsIn lytic cycle, lambda transcription occurs in three stages:

1. Early gene products establish the lytic cycles2. Middle gene products - Replication 3. Late gene products - Assembly of new virus particles and lysis of the cell

Lambda DNA uses host RNA polymerase for the synthesis of proteins to undergo lysogenic cycle

Early transcription starts from promoters PL and PR

Fate of the viral genome, to undergo lytic or lysogenic cycle is decided by the cro and cI gene products

Between PL and PR repressor gene cI is present and which inhibit the transcription of PL and PR and allow the phage to undergo lysogenic cycle

Transcription inhibition of PL and PR occurs when the cI protein binds to operator sequences of early genes .

So the phage undergo lysogenic cycle.

If the cro proteins are able to occupy the preferred regions of the operator first, then stimulates its further synthesis and blocks synthesis of the other protein.

Hence the lambda DNA undergo lytic cycle.

Early transcripts from PL and PR stops at tL and tR1

Site tR2 stops any transcripts that escape beyond tR1 Shift from Early to middle stage transcription is directed by N gene product expressed from PL.

N gene product is an anti-terminator

PL and PR transcripts extended to genes red, O and P which are important for middle stage.

When the enough cro gene products is accumulated, it prevents the transcription from PL and PR

Then gene Q products expressed from PR directs middle to late shift .

Gene Q products is another anti-terminator like gene N product.

So transcription of Late genes across the cos region takes place. results in many phage particles .

Both the gene N and gene Q are positive regulators essential for phage growth and lysis.

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