PHAGE M13

Attachment/Penetration:

Ff phages are 'male-specific', i.e. they require the F pilus on the surface of E. coli for infection. The first event in infection is an interaction between g3p, located at one end of the filament together with g6p, and the end of the F pilus.

This interaction causes a conformational changes in g8p: Initially, its structure changes from 100% alpha-helix to 85% alpha-helix and this causes the filament to shorten. The end of the particle attached to the F pilus flares open, exposing the phage DNA.

Subsequently, a second conformational change in the g8p subunits reduces its alpha-helical content from 85% to 50%, causing the phage particle to form a hollow spheroid about 40nm in diameter and expelling the phage DNA, thus initiating the infection of the host cell.

g8p is stripped off and ends up in the inner cell membrane, where it may possibly be stored and reused to produce new particles.

Genome Replication:

Infecting (+)strand DNA is converted in d/s 'RF' form by host cell enzymes, which together with g2p build up a pool of RF DNA in the cell. Virus proteins are synthesized from this pool of DNA.

Later in the infection, the concentration of g5p in the cell builds up. This protein binds to newly formed (+)strands, causing a switch from RF to (+)strand replication (coated with g5p).

Assembly/Release:

Assembly occurs at the inner membrane of the cell. DNA is extruded into the periplasmic space and the g5p coat is replaced by g8p. g5p is recycled to form further particles (efficient!).

These are temperate (i.e. non-lytic) phages - establish permanent infections without lysogeny. ~300 particles/cell/generation are produced; titres in infected cultures up to 5x1012/ml (~150mg/l) - good recombinant vectors.

...