Microbial Genetics (Micr340) Lecture 11 Phage and Lysogeny.
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Microbial Genetics (Micr340)
Lecture 11Phage and Lysogeny
Lysogeny: another life-style for some phages
Lysogenic phage – able to maintain a stable relationship with the host cell in which they neither multiply nor are lost from the cell.
In lysogenic state, the phage DNA is either integrated into host chromosome or replicates as a plasmid
The phage DNA in the lysogenic state is callled a prophage and the bacterium harboring a prophage is a lysogen for that phage
Phage
Phage : lytic development
Fairly large phage Three major stages of gene expression
during development First, genes N and cro. Genes involved with replication and
recombination Third, genes encoding head and tail
proteins and enzymes involved in cell lysis
Transcriptional antitermination
Transcription begins at the promoter but then soon terminates unless certain conditions are met.
Phage uses antitermination of regulation in at least two stages in its development The N protein regulates the synthesis of its
recombination and replication functions The Q protein regulates the synthesis of its
head and tail proteins
Transcriptional antitermination
Transcriptional antitermination
Antitermination sites
Phage DNA replication
Phage DNA is linear within its head It cyclizes after entering the cell Its cohesive ends, cos sites, are
important for cyclization
Lambda () Phage cos sites
cos sites (complementary overhang sites) or cohesive ends, consist of complementary 12 base pair overhangs, one on the 3’ end of each strand.
Note, wild type phage genome is about 48.5 kb long
(not drawn to scale)(not drawn to scale)
Phage DNA replication
Phage cloning vectors
Cosmids cos site Only head of DNA can be packaged if
flanked by cos sites Packaging extracts
Lysogenic vectors, retains phage ’s capability to integrate into host chromosome as a single copy DNA
Phage : lysogeny
The cII gene product Phage integration Maintenance of lysogeny Regulation of repressor synthesis Immunity to superinfection The induction of
Formation of lysogens
Integration of Phage DNA
Regulation of repressor synthesis
Induction of
Cro’s role in induction
Competition between lytic and lysogenic cycles
Competition between lytic and lysogenic cycles
Specialized transduction
In specialized transduction, only bacterial genes close to the attachment site of the prophage can be transduced. In contrast, generalized transduction, essentially any gene of the donor bacterium can be transferred.
In specialized transduction, phage carries both bacterial genes and phage genes. In contrast, generalized transduction phage carries only bacterial genes
Specialized transduction
Specialized transduction
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