Microbial Genetics (Micr340) Lecture 11 Phage and Lysogeny.

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