|Following the unveiling of the double helical DNA structure in the '50s and the elaboration of the Central Dogma in the '60s, the field of molecular genetics found itself in search of a technology that would allow us to examine the nucleotide sequences corresponding to a unit of heritable information.
The problem is that those nucleotide sequences are embedded in an enormous genome.
The development of recombinant DNA technology provided a solution to this isolation problem.
Recombinant DNA technology originated with the collaboration between biochemists and geneticists to understand the Central Dogma. A favorite model system of the time was the prokaryote E. coli and its attendant bacteriophage. Among the phenomena being studied was that of
Bacteriophage lambda Host Restriction
a lambda phage stock grown on one strain of E. coli (strain B) infects that strain with high efficiency.
This same phage stock infects a second E. coli strain R with very low efficiency (1 in 203 plaques grow)
Recovery and re-titreing of progeny phage from one of these rare plaques shows that they infect the R strain with high efficiency, but infect the B strain with very low efficiency (1 in 203 plaques grow)
Recovery and re-titreing of progeny phage from one of these rare plaques shows the same phenomena described above.
Phage grown in one strain infect that strain with high efficiency but infect the other strain with low efficiency.
|The detailed characterization of this phenomenon lead to a Nobel Prize (Arbor, Nathans & Smith 1978).
Host restriction turned out to be a bacterial analog of an immune system allowing bacteria to recognize and defend themselves against foreign invaders (DNA molecules).
Host restriction is a two component system consisting of
|1) a Restriction Endonuclease|
|Restriction endonucleases get their name from
i) the phenomenon of host restriction
ii) enzymatic cleavage of the DNA backbone within the DNA molecule (as opposed to exonucleases which only break off nucleotides from the ends of a linear DNA molecule)
When the phage injects its genome into an unfamiliar host on infection, the host's restriction enzymes recognize the phage DNA as foreign and break it into smaller linear pieces which are subsequently degraded by host exonucleases.
Well, thats a nice story, but how do these restriction enzymes recognize the host's genome as self and avoid degrading the heritable information store encoded therein?
|2) a Restriction Methylase
|The host restriction enzymes are paired with DNA modifiying enzymes which add methyl groups to the host genome and render it unrecognizable by the Restriction Endonucleases. Once modified, the genome is therefore protected from degradation.
When a phage is grown for several generations in a given host, the progeny phage acquire the methylation pattern specific to the hosts modification enzyme. These progeny phage evade the hosts 'immune system' since their genomes are not recognizable as foreign by the hosts restriction enzymes (it has the hosts self-methylation pattern).
When these progeny phage infect a second host strain, the phage genome is immediately recognized by the second host's restriction enzymes and degraded as it lacks the new hosts specific methylation pattern.
So where do the rare plaques come from (the 1 in 103 plaques that grow on the new host)?
We can envision two types of methylation events.
The first involves the methylation of host genome sequences after they have replicated.
|The secont type involve the methylation of completely unmethylated sites. Unmethylated sites may occur during very rapid cell division - but they are normally rare.
De-novo methylation of totally unmethylated sites is an extremely inefficient process. This accounts for the low efficiency of phage infection in a new host strain.
The majority of incoming phage genomes are recognized by the restriction endonucleases
(999 out of 1000) while only the rare infection (1 in 1000) is methylated before it is recognized by the host restriciton enzymes.
Once methylated however, all subsequent phage genomes are efficiently methylated at hemi-methylated sites on replication. This explains the high efficiency infection with the individual plaques that escaped restriction.