User:Braegan28/M13 bacteriophage

Edited writing from phage particle paragraph including adding citations that were missing. Also added last sentence of phage particles section as shown below and added some information on the life cycle. The final version is shown below and was transferred to M13 bacteriophage article.

Phage particles
The phage coat is primarily assembled from a 50 amino acid protein p8, which is encoded by gene 8 in the phage genome. For a wild type M13 particle, approximately 2700 copies of p8 are assembled into the 900 nm long coat. The coat's dimensions are flexible though, as the number of p8 copies adjusts to accommodate the size of the single stranded genome it packages. The phage appear to be limited to approximately twice the natural DNA content. However, deletion of a phage protein (p3) prevents full escape from the host E. coli, and phages that are 10-20X the normal length with several copies of the phage genome can be seen shedding from the E. coli host.

At one end of the filament are up to five copies of the surface exposed protein (p9) and the more buried companion protein (p7). If p8 forms the shaft of the phage, p9 and p7 form the "blunt" end that is seen in micrographs. These proteins are very small, containing only 33 and 32 amino acids respectively, though some additional residues can be added to the N-terminal portion of each, which are then presented on the outside of the coat. At the other end of the phage are five copies of the surface exposed (p3) and its less exposed accessory protein (p6). These form the rounded tip of the phage and are the first proteins to interact with the E. coli host during infection. Protein p3 is also the last point of contact with the host as a new phage buds from the bacterial surface. The production of phage particles causes a host cell to grow and divide, but it does not lead to lysis of the cell.

Life Cycle
Entry of the virus into a host cell is mediated by the p3 protein, specifically the N domains, binding to the primary and secondary receptors of the host cell. After the positive single strand DNA has entered the cell, it is duplicated to form the double stranded DNA that is then used to transcribe the mRNA that will build the proteins.