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Appendix IX
Bacteriophage λ
The mature λ map is a 48.6 kb long linear double-stranded DNA with 11-basepair overhanging cohesive
single-stranded tails on either end. The chromosome is divided into three general regions – left arm, right
arm, and central region. The right arm contains genes involved in replication and lysis and the right arm
contains genes for phage maturation. The central region controls lysogeny and superinfection immunity.
When λ enters a susceptible cell, the single-stranded tails anneal to generate a circle. The site formed by the
single stranded tails is called COS (cohesive site) and the left and right arms become contiguous and form a
single operon, the right hand operon. The central region comprises a second operon, the left hand operon.
λ is a temperate phage, meaning that upon entering a cell, the phage may enter either the lysogenic or lytic
cycle. During lysogeny, there is a site-specific recombination event between the λ att site and a primary
insertion site on the host chromosome adjacent to the lac operon. This event is mediated by the λ int gene.
There are other, secondary insertion sites elsewhere on the host chromosome. There are thus three possible
maps for λ, the linear mature map, the circular vegetative map, and a second linear map. In the mature map
the ends are flanked by the COS site while in the lysogenic map the ends are flanked by the att site.
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Appendix IX
This site specific recombination event is reversible. If a lysogen is presented with DNA damage, in the form
of ultraviolet light, for example, the int gene together with the xis gene perform the reverse site-specific
recombination event and excise the phage from the host genome, restoring it to the circular vegetative map,
and the phage then enters the lytic cycle. This process is called induction. Apart from DNA damage,
spontaneous induction occurs at low levels.
During the lytic phase, λ undergoes two replication phases, early and late. In the early phase, λ replicates via
the Cairns or theta mode to produce multiple circular genomes. During late phase, the phage switches to
rolling circle or sigma mode to produce mature linear phage genomes. Packaging is performed by a “headfull” process in which DNA is inserted into a pre-formed head. This imposes certain size restrictions that
impact the use of λ as a cloning vector:
maximum DNA length
minimum DNA length
kilobases
52.0
38.5
Percent of Genome Length
105
78
size of left + right arms
29.1
60
The mature λ map is a 48.6 kb long linear double-stranded DNA with 11-basepair overhanging cohesive
single-stranded tails on either end. The chromosome is divided into three general regions – left arm, right
arm, and central region. The right arm contains genes involved in replication and lysis and the right arm
contains genes for phage maturation. The central region controls lysogeny and superinfection immunity.
When λ enters a susceptible cell, the single-stranded tails anneal to generate a circle. The site formed by the
single stranded tails is called COS (cohesive site) and the left and right arms become contiguous and form a
single operon, the right hand operon. The central region comprises a second operon, the left hand operon.
λ is a temperate phage, meaning that upon entering a cell, the phage may enter either the lysogenic or lytic
cycle. During lysogeny, there is a site-specific recombination event between the λ att site and a primary
insertion site on the host chromosome adjacent to the lac operon. This event is mediated by the λ int gene.
There are other, secondary insertion sites elsewhere on the host chromosome. There are thus three possible
maps for λ, the linear mature map, the circular vegetative map, and a second linear map. In the mature map
the ends are flanked by the COS site while in the lysogenic map the ends are flanked by the att site.
146
Bacteriophage λ
This site specific recombination event is reversible. If a lysogen is presented with DNA damage, in the
form of ultraviolet light, for example, the int gene together with the xis gene perform the reverse sitespecific recombination event and excise the phage from the host genome, restoring it to the circular
vegetative map, and the phage then enters the lytic cycle. This process is called induction. Apart from
DNA damage, spontaneous induction occurs at low levels.
During the lytic phase, λ undergoes two replication phases, early and late. In the early phase, λ replicates
via the Cairns or theta mode to produce multiple circular genomes. During late phase, the phage switches
to rolling circle or sigma mode to produce mature linear phage genomes. Packaging is performed by a
“head-full” process in which DNA is inserted into a pre-formed head. This imposes certain size
restrictions that impact the use of λ as a cloning vector:
maximum DNA length
minimum DNA length
kilobases
52.0
38.5
Percent of Genome Length
105
78
size of left + right arms
29.1
60
147
Appendix IX
Source: New England BioLabs
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