Mini M13 bacteriophage: circular fragments of M13 DNA are replicated and packaged during normal infections

M13 filaments, 0.2–0.5 of the standard length consitituted 20% of a large stock of phages obtained after 8 passages beyond the initial single-plaque isolation. The mini particles, purified by sucrose velocity sedimentation, contain circular, single-strand DNA of 0.2–0.5 the normal length. The particles are reproduced only upon coinfection with normal M13. Infection with an equal number of mini and full-size M13 particles generated a majority of mini forms among the intracellular DNAs (duplex and single-stranded circles) and the released phage particles. Analysis of mini phage DNA by heteroduplex electron microscopy showed them to be fragments of the M13 circle containing the unique site for a restriction endonuclease and extending for a variable distance (about 0.2–0.5 unit length) in one direction from this site.