P1 Phage - Genome


The genome of the P1 phage is moderately large, around 93Kbp in length (compared to the genomes of e.g. T4 - 169Kbp, lambda - 48Kbp and Ff - 6.4Kbp). In the viral particle it is in the form of a linear double stranded DNA molecule. Once inserted into the host it circularizes and replicates as a plasmid.

In the viral particle the DNA molecule is longer (110Kbp) than the actual length of the genome. It is created by cutting an appropriately sized fragment from a concatemeric DNA chain having multiple copies of the genome. (see the section below on lysis for how this is made) Due to this the ends of the DNA molecule are identical. This is referred to as being "terminally redundant". This is important for the DNA to be circularized in the host. Another consequence of the DNA being cut out of a concatemer is that a given linear molecule can start at any location on the circular genome. This is called a cyclical permutation.

The genome is especially rich in Chi sequences recognized by the bacterial recombinase RecBCD. The genome contains two origins of replication, oriR which replicates it during the lysogenic cycle and oriL which replicates it during the lytic stage. The genome of P1 has 112 protein coding and 5 untranslated genes. It even encodes 3 of its own tRNAs which are expressed in the lytic stage.

The gene that encodes the tail fibers have an set of sequences that can be targeted by a site specific recombinase Cin. This causes the C terminal end of the protein to switch between two alternate forms at a low frequency. The viral tail fibers are responsible for the specificity of binding to the host receptor. The targets of the viral tail fibers are under a constant pressure to evolve and evade binding. This method of recombinational diversity of the tail allows the virus to keep up with the bacterium. This system has close sequence homologies to recombinational systems in the tail fibers of unrelated phages like the mu phage and the lambda phage.

Read more about this topic:  P1 Phage

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