Genetics of Bacteriophages. 1. Lytic growth of bacterio phages. 2. Morphology of selected bacteriophages. 3. Scaffolded assembly. Assembly of complex phages is assisted by scaffolding proteins. These are removed once assembly is complete. 4. ϕ X174.
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Genetics of Bacteriophages
Morphology of selected bacteriophages
The phage fX174 is an icosahedral phage that contains a circular single-stranded
DNA molecule of 5386 nucleotides. It codes for 11 proteins, each of which has been identified. Adding together the size of all those proteins comes to 2330 amino acids, which would require 6990 nucleotides (3 2330) – substantially more than the total length of the genome
Firstly the genes are very tightly packed – there is very little non-coding
sequence in the genome. In most cases, the end of one gene is directly adjacent to (or slightly overlaps with) the start of the next.
Secondly, one of the proteins (A*) corresponds to the C-terminal region of protein A
M13, represent another type of single-stranded phage. These are ‘male-specific’ phages, since they infect the cell by attaching to the tips of the pili specified by the F plasmid
As the replication cycle proceeds, one of the phage proteins that is produced is able to bind to the single-stranded DNA and divert it into the production of phage particles by targeting it to the cell membrane
where it is extruded from the cell, with phage coat proteins being polymerized around it during its passage through the membrane
Another type of male-specific phage is represented by MS2
This is an icosahedral RNA-containing phage which attaches to the sides of the F-pili, rather than to the tip as M13 does.
It is an extremely simple phage, containing some 3600 nucleotides ,coding for just three genes: a coat protein, a maturation protein and a replicase
Terminal redundancy of bacteriophage T4. Multiple length linear DNA is the substrate for packaging into phage particles. The amount of DNA packaged in longer than the genome size, leading to terminal redundancy (a sequence of about 1600 base pairs at one end of the molecule is repeated in the same orientation at the other end)
Replication of bacteriophage lambda DNA. After infection, the cohesive ends are ligated to form a circular molecule which replicates (theta mode) to generate more circular DNA. Later, replication switches to the rolling circle mode, generating multiple length linear DNA for packaging into phage particles
LysogenyStructure of the attachment sites of l. (a) Integration requires site-specific recombination between attP (on the phage) and attB (on the chromosome). (b) After integration, the sites at either end of the prophage (attL and attR) have the same core sequence (shaded) as attP and attB but different flanking sequences
Control of the lytic/lysogenic switch in λ. CII stimulates transcription of the cI repressor gene from PE; CIII stabilizes CII. CI represses PL and PR and stimulates PM allowing further synthesis of repressor. Cro represses PL, PR and PM