Gene Regulation: Lambda Phage

Question 1

Lambda phage fact file

Answer 1

Most abundant biological entity on earth
50 million phage particles per ml of sea water
Only infects E. coli
Circular dsDNA, icosahedral head of 50-60nm diameter. Has a set of flexible tail fibres (150nm)
Tail fibres + J protein allow phage to bind to host surface
Has lytic and lysogenic lifestyles

Question 2

Lambda transcription

Answer 2

Driven by 2 Leftward and Rightward Promoters (Pl + Pr), which are responsible for expression of nearly all of lambda’s genes
Genome size: ~49kbp, with 70 lambda genes
Has divergent orientation, Pl drives expression of genes encoded on left hand side + Bottom strand, whereas Pr drives expression on right hand side + top strand
Genes under Pl + Pr are expressed in a regulatory cascade of 3 temporal phases, with the genes closest to the promoters expressed first (immediate-early genes, they are cro + N genes in lambda)
In early lytic infection, transcription stops at TL1 and TR1 (terminators)

Question 3

N protein

Answer 3

Disables terminators TL1 + TR1 to allow transcription of delayed-early genes further along the phage genome, representing the 2nd cascade of expression

Question 4

Regulating Cascade in Lambda phage

Answer 4

Immediate-early genes encode important regulators required for later stages of the lytic pathway
Delayed-early genes encode genes required for phage genome replication, recombination + gene regulation
Delayed-late genes encode proteins required for tails + head of the phage particles + cell wall lysis

Question 5

Promoters and their specific gene expressions

Answer 5

Pl = Specifies the N proteins that acts to dampen activity TL1 and TR1 activity
PRM = Specifies lambda repressor, encoded by the C1 gene
Pr = Specifies the cro repressor

Question 6

Operator elements in lambda phage

Answer 6

Rightward Operator (Or) and Leftward Operator (Ol), they regulate Pr + Pl expression respectively in response to cro + lambda repressor activity

Question 7

Pr Structure

Answer 7

Near to Prm, which together share Or in a divergent arrangement
Or is made of 3 palindromic 17bp sites that are adjacent to one another, together displaying 2-folded inverted symmetry (Or1, Or2, Or3)
Occupation of Or1 + 2 by lambda repressor will cause repress expression of cro + all other Rightward gene products at later cascade stages
Occupation of Or3 will cause block C1 gene expression