Colicin E4-CT9 is proteolytically degraded after discharge from producing cells in liquid cultures.

Colicin E4 was produced in very large amounts when Escherichia coli K12 strain W3110 pColE4-CT9 was grown in the presence of 0.5 microgram mitomycin C ml-1. The colicin was discharged from the producing cells when they lysed and was then degraded by a protease located on the surface of the producing cells. Colicins and proteolytic fragments derived from them could be concentrated from spent culture medium by filtration through Millipore membrane filters. Colicins are probably retained by these filters by hydrophobic interactions since binding was unaffected by changes in ionic conditions but was completely inhibited in the presence of ionic or non-ionic detergents.