LAMBDA PHAGE AND THEIR REPRODUCTION

Introduction, Structure, Lambda DNA, Life cycle of Lambda phage, Genetic switch in Lambda phage

1. REPRODUCTION OF LAMBDA PHAGE K.K.Banumathi B.Sc.Microbiology

2. INTRODUCTION Lambda phage is a bacteriophage. Bacteriophages especially infect bacteria , hence the name. Lambda phage very effectively uses K12 strain of Escherichia coli . Another name of lambda phage is Temperate phage which means it act as both virulent and non-virulent. If the phage act as virulent ,it follow lytic cycle.(Lytic-Lysis) The phage act as non-virulent, it follow lysogenic cycle.

3. Structure of lambda phage Lambda phage belongs to Sophoviridae family. Head is icosadhedral in shape and is about 55nm in diameter. Capsid contains 300-600 subunits of capsomers. Molecular weight of capsid protein is 37,500D. Head is joined to tail by head tail connector. Tail is long, flexible and is about 150 × 8nm in size with 25nm long non-contractile tail fiber.

4. Lambda DNA DNA is double stranded linear and 17 µm in length. Genome has 46,500 base pairs. The molecular weight of DNA is 31 ×10 6 Daltons. Lambda DNA possesses 12 nucleotides as extending units in either side of the DNA, which are complementary to each other, & it is said to be cohesive ends.

5. Life cycle of lambda phage

6. Lifecycle of lambda phage Lambda phage is a temperate phage and is capable carrying out both lytic and lysogenic life cycle. Lytic cycle Attachment Entry Transcription & Translation of early gene Breakdown of Bacterial chromosome Synthesis of structural proteins Replication of DNA

7. Assembly or Morphogenesis Release Attachment Attachment of a phage on a surface of bacterium is called adsorption. Entry The penetration of phage DNA into the bacterium is called entry. Penetration of DNA takes place by a pushing force created during tail contraction. The phage produce Lysozyme, which dissolves the bacterial cell wall and drills a hole in it.

8. Transcription and Translation of early gene Early phage genes are transcribed into mRNA and translated into proteins. Products of early genes are nuclease enzyme, Host DNA & RNA polymerase modifying enzyme. Breakdown of bacterial chromosome Super Helical DNA becomes relaxed due to helix destabilizing proteins of phage. Nucleases digest the bacterial DNA into free nucleotides. The free nucleotides remains in the cytoplasm and used or phage replication.

9. Synthesis of structural proteins The phage utilizes host machinery for the synthesis of capsid, tail, tail fibres , core and phage DNA replication. Structure proteins are the products of late genes of phage. This results in accumulation of viral components in bacterial cell cytoplasm. Replication of DNA Phage DNA replication begins at the 6 th minute of infection. The phage DNA has unusual nucleotide 5-hydroxy methyl cytosine (HMC). It is synthesized from the cytosine residue of host DNA.

10. DNA synthesis begins with the synthesis of RNA primer. Then DNA polymerase proceed the DNA synthesis by the addition of new nucleotides using RNA primer as template. At the end of this step, phage of DNA molecules accumulate in the host cell. Assembly or Morphogenesis Build up of virus by fitting the viral proteins & DNA together is called viral assembly. Release This occurs 22 minutes after viral attachment to the host cell. Lysozyme synthesized by virus breaks cell wall, leading

11. to cracks in the cell wall. Lysis of host cell releases 150-300 progeny virion. Lysogenic cycle Attachment Entry Circularization and To form prophage Reproduction of Lysogenic bacterium Enters into lytic cycle Attachment and Entry This steps happen as same as mentioned in the lytic cycle.

12. Circularization and To form prophage After the penetration of phage DNA , the DNA get circularized within the bacterial (host) cell. Then it will be fused (mingled) with the bacterial chromosome. These fusion of viral and bacterial DNA is called as Prophage . Reproduction of lysogenic bacterium The infected bacterium i.e., lysogenic bacterium reproduces normally with prophage. Enters into lytic cycle The lysogenic bacterium become virulent at their favourable condition. It follow lytic cycle.

13. Occasionally, the prophage excise from the bacterial chromosome and enters the lytic cycle. This point of cycle switch is known as Genetic switch Genetic switch in lambda phage Genetic switch is nothing but the swap of lytic into lysogenic (or) lysogenic into lytic cycle.

14. The genome of lambda phage is madeup of 2 important thing i.e., cro protein[Lytic cycle] and CI region (lambda repressor)[Lysogenic cycle]. Lytic cycle The cro protein region is composed of operator 3 (OR3). The OR3 binds with CI region. Then the CI region can’t able to transcribe. So the lysogenic cycle doesn’t takes place. Lytic cycle is carried into left side. Lysogenic cycle The CI region is composed of operator 1,2 (OR1,2). The OR1,2 binds with cro protein region. Then the cro region

15. can’t able to transcribe. So, the lytic cycle doesn’t take place. Lysogenic cycle is carried into right side.