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Bacteriocins were first discovered by A. Gratia in 1925 and are proteinaceous toxins that are produced by bacteria to inhibit the growth of closely related bacterial strains [1]. Since the bacteriocins that he found, only killed Escherichia coli, he called them colicins. Later on, it became clear that colicins are part of the broad class: the bacteriocins. So colicins[2] are proteins that are produced by E. coli strains that have the colicinogenic plasmid and are toxic for E. coli strains without this plasmid. The colicinogenic plasmid contains the genes for the synthesis and release of the colicins. Colicins are released into the environment to reduce competition from other related bacterial strains. The mechanism through which colicins translocate to the cytoplasm membrane, is by binding to the outer membrane receptors. Once they reach the cytoplasmic membrane they depolarize the cytoplasmic membrane, induce DNase acitivty, RNase activity and possibly even inhibit murein synthesis. Colicins are organized in three domains that are each involved in a different step of the process of exterminating related bacteria. More information on bacteriocins and colicins can be found on the BACTIBASE open-access database [3,4].

Colicin DNases are metal-dependent enzymes that randomly degrade DNA [5]. Four DNase colicins have been reported ColE2, E7, E9 and E9 [7, 8] of which ColE2 was the first reported. The DNase domains of these colicins share 65% sequence identity. The DNA sequence of the entire colicin E2 operon is determined and exists of the colicin activity gene (ceaB), the colicin immunity gene (ceiB), and the lysis gene (celB), which is essential for colicin release from producing cells.

Colicin E2 (E2) is a protein antibiotic synthesized by certain strains of coliform bacteria that carry the ColE2 plasmid and it is classified in the E group of colicins along with colicins El and E3. It was first discovered that E2 causes degradation of DNA, while colicin E3 (E3) causes specific inhibition of protein synthesis in sensitive E. coli cells treated with these colicins [9].

Shown below in Figure 1 [to be added] is the sequence alignment of the R- and C-domain junction of the nuclease E colicins along with a portion of the R-domain. DNase colicins E2, E7, E8, and E9 have almost identical R-/C-domain linker regions. Compared with colicin E7, the sequence identity of the other DNase colicins in the linker region is 27/30 for E2, E8, and E9. the linker regions of colicins E3, E4, and E6 have 12/30, 13/30, and 13/30 residues identical to colicin E7.

References

[1] http://en.wikipedia.org/wiki/Bacteriocins
[2] Eric Cascales et al., Colicin Biology , Microbiology and Molecular Biology Reviews, March 2007, p. 158-229, Vol. 71, No. 1.
[3] "Kleanthous Research Group". University of York. Retrieved 11 April 2011.
[4] Hammami R, Zouhir A, Ben Hamida J, Fliss I (2007). "BACTIBASE: a new web-accessible database for bacteriocin characterization". BMC Microbiology 7: 89.
[5] James, R., C. N. Penfold, G. R. Moore, and C. Kleanthous. 2002. Killing of E. coli cells by E group nuclease colicins, Biochimie 84:381–38.
[6] Schaller, K., and M. Nomura; 1976, Colicin E2 is an endonuclease; Proc. Natl. Acad. Sci. USA 73:3989–3993.
[7] Chak, K. F., W. S. Kuo, F. M. Lu, and R. James. 1991. Cloning and characterization of the ColE7 plasmid; J. Gen. Microbiol. 137:91–100.
[8] Cooper, P. C., and R. James. 1984. Two new colicins, E8 and E9, produced by a strain of Escherichia coli. J. Gen. Microbiol.130:209-215.
[9] Nomura, M. (1963) Cold Spring Harbor Symp. Quant. Biol. 28, 315-324.