Colicins

Coiicin E (from E.coli) [11032-88-5], Purified by salt extraction of extracellular-bound colicin followed by salt fractionation and ion-exchange chromatography on a DEAE-Sephadex column, and then by CM-Sephadex column chromatography [Schwartz and Helinski J Biol Chem 246 6318 1971],... 

Colicins are pore-forming proteins, produced by certain strains of E. coli, that kill or inhibit the growth of other, competing bacteria and even other strains of E. coli (a process known as allelopathy). Channel-forming colicins are released as soluble monomers. Upon encountering a host cell, the colicin molecule traverses the bacterial outer membrane and periplasm, then inserts itself... 

Interestingly, certain other pore-forming toxins possess helix-bundle motifs that may participate in channel formation, in a manner similar to that proposed for colicin la. For example, the S-endotoxui produced by Bacillus thuringiensis is toxic to Coleoptera insects (beetles) and is composed of three domains, including a seven-helix bundle, a three-sheet domain, and a /3-sandwich. In the seven-helix bundle, helix 5 is highly hydrophobic, and the other six helices are amphipathic. In solution (Figure 10.32), the six amphipathic... 

Parker, M., Tucker, A., Tsernoglon, D., and Pattns, F., 1990. Insights into membrane insertion based on studies of colicins. Trends in Biochemical Sciences 15 126-129. 

The body possesses a normal bacterial flora which, by competing for essential nutrients or by the production of inhibitoiy substances such as monolactams or colicins, suppresses the growth of many potential pathogens. 

Mainly the outer membrane ferrichrome receptor and transporter FhuA will be discussed because most structural and functional studies have been performed with this protein. In fact, FhuA was the first outer membrane protein identified (called TonA), with known functions as a phage and colicin receptor, that are related to iron transport (for a historical account, see Braun and Hantke 1977). 

Lazdunski, C. J. (1995). Colicin import and pore formation a system for studying protein transport across membranes Mol. Microbiol., 16, 1059-1066. 

Stroud, R. M., Reiling, K., Wiener, M. and Freymann, D. (1998). Ion-channel-forming colicins, Curr. Opin. Struct. Biol., 8, 525-533. 

Smarda, J. and Smajs, D. (1998). Colicins - exocellular lethal proteins of Escherichia coli, Folia Microbiol. (Praha), 43, 563-582. 

Control of action difficult Colicin lytic genes... 

The VP1 helices could then insert into the membrane and form a pore, which could allow the passage of RNA through the lipid bilayer into the cytosol. This is reminiscent of the pore formation by colicin [51]. 

Drury, L.S. Buxton, R.S. Identification and sequencing of the Escherichia coli cet gene which codes for an inner membrane protein, mutation of which causes tolerance to colicin E2. Mol. Microbiol., 2, 109-119 (1988)... 

E3 Colicin Inhibits protein synthesis by destroying mRNA Diaz et al. (1994)... 

Channel-forming toxins and antibiotics. Some of the bacterial toxins known as colicins (Box 8-D) kill susceptible bacteria by creating pores that allow K+ to leak out of the cells. One part of the complement system of blood (Chapter 31) uses specific proteins to literally punch holes in foreign cell membranes. Mel-litin, a 26-residue peptide of bee venom,372 373 as well as other hemolytic toxins and antibiotic peptides of insects, amphibians, and mammals (Chapter 31) form amphip-athic helices which associate to form voltage-dependent anion-selective channels in membranes.374-377... 

The N-terminal portion of the 522-residue polypeptide chain of colicin El appears to be required for transport into the membrane and the central part for binding to the receptor the channelforming property is characteristic of the C-terminal region.k A similar organization has been established for the smaller colicin N translocation domain, (residues 1-66), receptor domain, (residues 67-182), and pore-forming domain (residues 183-387). 

The colicin El plasmid is a 4.43 MDa circular double stranded DNA molecule consisting of 6646 base pairs.1 Only one site is susceptible to cleavage by the restriction endonuclease ECoRl (Chapter 26) This feature has led to its widespread use in cloning of genes. 

Not all hereditary traits follow the Mendelian patterns expected for chromosomal genes. Some are inherited directly from the maternal cell because their genes are carried in the cytoplasm rather than the nucleus. There are three known locations for cytoplasmic genes the mitochondria, the chloroplasts, and certain other membrane-associated sites.285 286 An example of the last is found in "killer" strains of yeast. Cells with the killer trait release a toxin that kills sensitive cells but are themselves immune. The genes are carried in double-stranded RNA rather than DNA, but are otherwise somewhat analogous to the colicin factors of enteric bacteria (Box 8-D). Similar particles (kfactors) are found in Paramecium.287... 

Antisense RNA. Another mechanism of control of either transcription or of plasmid replication involves small molecules of RNA that are transcribed from the opposite strand than the template strand used for mRNA synthesis.1 13 165-166b These antisense RNA molecules have at least some part of their sequence complementary to that of the mRNA and to the corresponding sequence in DNA. A well-studied example is control of the copy number of the colicin El and other plasmids of E. co/i.167-169 Two transcripts, RNAI and RNAII, are initiated upstream from the origin of replication (Fig. 28-8). RNA II is a 555-nucleotide primer of replication. It is synthesized as a longer transcript that is cut by RNase H at ori. This... 

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