Structure and function of periplasmic chaperone-like proteins involved in the biosynthesis of K88 and K99 fimbriae in enterotoxigenic Escherichia coli

The nucleotide sequence of faeE and fanE, two genes involved in the biosynthesis of K88 and K99 fimbriae, respectively, was determined and the amino acid sequence of the FaeE and FanE proteins was deduced. Immunoblotting of subcellular fractions with an antiserum raised against purified FaeE confirmed that FaeE is located in the periplasm. Indications were obtained that FaeE functions as a chaperone-like protein. Its interaction with the fimbrial subunit (FaeG) in the periplasm stabilized this polypeptide and prevents its degradation by the cell-envelope protease DegP. Furthermore, FaeE prevents the formation of FaeG multimers which cannot be incorporated into fimbriae. The reactions of the FaeE/FaeG dimers with a set of monoclonal antibodies directed against the various epitopes present on K88 fimbriae revealed that the fimbrial subunits associated with FaeE were present in a conformation resembling their native configuration. Indications about the domains in FaeG involved in the interaction with FaeE are discussed.

Functioning of the stable signal peptide of the pCloDF13-encoded bacteriocin release protein

The pCloDF13-encoded bacteriocin release protein (BRP) is a lipoprotein which is synthesized as a precursor with an amino-terminal signal peptide that appears to be stable after cleavage. The role of the stable signal peptide in the functioning of the BRP was studied with respect to the release of cloacin DF13, 'lysis' and leakage of periplasmic proteins. The BRP gene fragment encoding the stable signal peptide was replaced by a fragment encoding the unstable peptide of the murein lipoprotein (Lpp). The resulting hybrid protein was normally acylated and processed by signal peptidase II, leaving no stable signal peptide in the cells. Expression of the hybrid protein did not result in the specific release of cloacin DF13, whereas 'lysis' and the release of periplasmic enzymes were unaffected. These results indicated a role for the stable BRP signal peptide in the translocation of cloacin DF13 across the cytoplasmic membrane.

Structure, localization and function of FanF, a minor component of K99 fibrillae of enterotoxigenic Escherichia coli

The DNA sequence of the K99 fanF gene, encoding FanF, was determined. An open reading frame of 999 bp was found. The primary structure of FanF was deduced and analysis revealed the presence of a signal sequence of 22 amino acid residues. The mature protein contains 311 amino acid residues (Mr 33,905 D). The amino acid sequence of FanF showed similarity with the K88ab major subunit FaeG. A specific mouse antiserum against FanF was prepared by constructing and purifying a hybrid Cro-LacZ-FanF protein. Minicell analysis, immunoblotting and immunoelectronmicroscopy revealed a pool of FanF in the periplasm of K99-producing cells and showed, furthermore, that FanF is a minor component of K99 fibrillae, present at the top and in or along the shaft of the K99 fibrillar structures. A fanF mutant plasmid was constructed. Cells harbouring this plasmid produced all K99-specific proteins, except FanF, but produced 0.1% of the K99 fibrillae relative to 'normal' K99-producing cells. Electron microscopic observations showed that cells defective in fanF produce only a few (apparently short) K99 fibrillae. FanF, therefore, was supposed to play a role in initiation and elongation of K99 fibrillae formation. Thin-layer chromatography experiments involving purified receptor material showed that FanF is not required for binding of K99 fibrillae to the ganglioside receptor. Fibrillae produced by an adhesion-negative strain carrying a mutation in the K99 major fibrillar subunit were shown to contain a normal amount of FanF.

K88 fimbriae as carriers of heterologous antigenic determinants

The K88 fimbriae of enterotoxigenic Escherichia coli are strongly immunogenic antigens that can be used to evoke protective immunity. To find out whether these fimbriae can be used as carriers for foreign epitopes, a highly variable region present in the primary structure of the different K88 variants was replaced with five different heterologous epitopes to investigate to what extent these insertions affected the expression, assembly (biogenesis), stability and immunogenic properties of the resulting hybrid fimbriae. Amino acid residues 163-173, were replaced using site-directed in vitro mutagenesis and the hybrid fimbriae were tested for these aspects using ELISA, immunoelectronmicroscopy and immunoblotting. Replacement of this highly variable region did not affect the biosynthesis of fimbriae, although all mutations tested resulted in a reduced expression depending on the epitope inserted. Testing of the different hybrid fimbriae with a panel of monoclonal antibodies raised against the various K88 serotypes K88ab, K88ac and K88ad indicated that replacement of amino acid sequence 163-173 did not affect conserved or K88ab specific epitopes but the K88ac and K88ad specific conformation was lost. Immunization with hybrid fimbriae raises antibodies specific for the inserted heterologous epitopes.

The penultimate tyrosine residue of the K99 fibrillar subunit is essential for stability of the protein and its interaction with the periplasmic carrier protein

The role of the penultimate and conserved tyrosine residue of the K99 major fibrillar subunit (FanC) in fibrillae biosynthesis and functioning was investigated. By using oligonucleotide-directed in vitro mutagenesis the TAT codon of tyrosine-158 of fanC was changed into a TAG stop codon. The mutant fanC gene encoded a truncated major subunit lacking the two carboxyl-terminal amino acid residues. Furthermore, the tyrosine residue (position 158) was replaced by a serine residue or by a glutamic acid residue. The effect of these mutations on the expression and binding capacity of K99 fibrillae was investigated by using an ELISA, an haemagglutination assay, Escherichia coli minicells and suppressor strains. All mutations completely blocked K99 fibrillae biosynthesis and haemagglutination activity. The mature form of the truncated mutant FanC polypeptide could not be detected in minicells, but its precursor was expressed at a normal level. The results showed that the penultimate tyrosine residue is essential for the expression of mature fibrillar subunits and suggested a function in the interaction with the periplasmic transport protein FanE.

pCloDF13-encoded bacteriocin release proteins with shortened carboxyl-terminal segments are lipid modified and processed and function in release of cloacin DF13 and apparent host cell lysis

By oligonucleotide-directed mutagenesis, stop codon mutations were introduced at various sites in the pCloDF13-derived bacteriocin release protein (BRP) structural gene. The expression, lipid modification (incorporation of [3H]palmitate), and processing (in the presence and absence of globomycin) of the various carboxyl-terminal shortened BRPs were analyzed by a special electrophoresis system and immunoblotting with an antiserum raised against a synthetic BRP peptide, and their functioning with respect to release of cloacin DF13, lethality, and apparent host cell lysis were studied in Sup-, supF, and supP strains of Escherichia coli. All mutant BRPs were stably expressed, lipid modified, and processed by signal peptidase II, albeit with different efficiencies. The BRP signal peptide appeared to be extremely stable and accumulated in induced cells. Full induction of the mutant BRPs, including the shortest containing only 4 amino acid residues of the mature polypeptide, resulted in phospholipase A-dependent and Mg2+-suppressible apparent cell lysis. The extent of this lysis varied with the mutant BRP used. Induction of all mutant BRPs also prevented colony formation, which appeared to be phospholipase A independent. One shortened BRP, containing 20 amino acid residues of the mature polypeptide, was still able to bring about the release of cloacin DF13. The results indicated that the 8-amino-acid carboxyl-terminal segment of the BRP contains a strong antigenic determinant and that a small segment between amino acid residues 17 and 21, located in the carboxyl-terminal half of the BRP, is important for release of cloacin DF13. Either the stable signal peptide or the acylated amino-terminal BRP fragments (or both) are involved in host cell lysis and lethality.

Detection and identification of FaeC as a minor component of K88 fibrillae of Escherichia coli

A tribrid gene containing ompF, faeC, and lacZ sequences was constructed by subcloning a large central segment of the K88ab gene encoding the fibrillar subunit-like protein FaeC into the open reading frame expression vector pORF2. The resulting tribrid protein was isolated and used to raise antibodies against the FaeC protein. These antibodies were then used for the detection and subcellular localization of the FaeC protein in Escherichia coli harbouring the K88ab-encoding plasmid pFM205 or mutant derivatives. Immunoblotting of subcellular fractions and of purified fibrillae, and agglutination experiments using whole cells revealed that the FaeC protein is present in the periplasm and as a minor component in the K88ab fibrillae. FaeC was also detected in purified K88ac and K88ad fibrillae. Immunoelectron microscopy confirmed the presence of FaeC in K88ab fibrillae, particularly at the tips of the longer fibrillae.

Cloning, expression and release of native and mutant cloacin DF13 immunity protein

The pCloDF13 encoded immunity protein gene was subcloned in the expression vector pINIIIA1 and several deletion, insertion and point mutations were constructed in the amino-terminal and carboxyl-terminal regions of the protein. The expression, stability, BRP-dependent export and protective capacity of the native and mutant immunity proteins were studied by SDS-PAGE, immunoblotting and an in vivo activity assay. In the absence of cloacin the unbound, native immunity protein was stable produced by E. coli cells and released after BRP induction. The expression of most of the mutant immunity proteins was strongly reduced and non of the proteins were found to be released. All mutations in the carboxyl-terminal region strongly affected expression of the proteins, probably by causing protein instability and proteolytic degradation. One of these mutant immunity proteins, with an insertion mutation in its carboxyl-terminal region, still caused an intermediate immunity of susceptible cells against extracellularly added cloacin DF13. Mutations in the amino-terminal region of the immunity protein had less effect on its expression and did not affect the protective capacity of the protein.

Functioning of a hybrid BRP-beta-lactamase protein in the release of cloacin DF13 and lysis of Escherichia coli cells

The gene encoding a hybrid BRP-Bla protein consisting of the pCloDF13 encoded BRP signal sequence, 25 of the 28 amino acid residues of the mature bacteriocin release protein (BRP) and the mature portion of beta-lactamase (Bla) was subcloned in the expression vector pEB112. A similar construct was made using a mutant gene encoding a BRP-Bla protein in which the cysteine residue at the +1 position was changed into a glycine residue. The expression, processing, functioning and subcellular localization of the 'wild-type' and mutant hybrid protein at high-level expression conditions were studied. The 'wild-type' BRP-Bla protein was mainly found in the outer membranes and possessed all the activities of the BRP itself; the protein was able to bring about the release of cloacin DF13 and caused apparent cell-lysis after high-level synthesis. The mutant hybrid protein was predominantly located in the inner membranes, was inactive in the release of cloacin DF13, but caused apparent cell-lysis only after strong induction.

Alterations in the carboxy-terminal half of cloacin destabilize the protein and prevent its export by Escherichia coli

Several overlapping carboxy-terminal and internal deletions were constructed in the cloacin structural gene. The expression, the binding of the cloacin DF13 immunity protein and the release into the culture medium of the mutant cloacin polypeptides were studied by immunoblotting and ELISAs. Minor alterations at the carboxy-terminal end of the cloacin did not affect protein expression, stability or release to a large extent, but larger carboxy-terminal deletions strongly destabilized the protein and no release was observed. The removal of a particular region within the carboxy-terminal portion of cloacin strongly destabilized the polypeptide and made it a target for proteolytic degradation. Binding of immunity protein did not affect stability and release of the mutant polypeptides. By using immunoelectron microscopy, the polypeptides that were not exported were located in the cytoplasm of producing cells. Large aggregates of these mutant polypeptides were not observed in the cytoplasm: the polypeptides were present in a soluble form.

Effect of a mutation preventing lipid modification on localization of the pCloDF13-encoded bacteriocin release protein and on release of cloacin DF13

The pCloDF13-encoded bacteriocin release protein (BRP; Mr 2,871) is essential for the translocation of cloacin DF13 across the cell envelope of producing Escherichia coli cells. Overproduction of this BRP provokes lysis (quasilysis) of cells. Construction and analysis of a hybrid BRP-beta-lactamase protein (BRP-Bla) demonstrated that the BRP contains a lipid modified cysteine residue at its amino terminus and is mainly located in the outer membrane. The significance of lipid modification for the localization and functioning of the BRP was investigated. Site-directed mutagenesis was used to substitute the cysteine residue for a glycine residue in the lipobox of the BRP and the BRP-Bla protein. The mutated BRP was unable to bring about the release of cloacin DF13 and could not provide the lysis (quasilysis) of host cells. However, the mutated BRP strongly inhibited the colony-forming ability of the cells, indicating that induction of the mutated protein still affected cell viability. In contrast to the wild-type BRP-Bla protein, the mutated BRP-Bla protein was mainly located in the cytoplasmic membrane, indicating that the mutation prevented the proper localization of the protein. The results indicated that lipid modification of the BRP is required for its localization and release of cloacin DF13, but not for its lethality to host cells.

Genetic organization and biogenesis of adhesive fimbriae of Escherichia coli

The genetic organization of the determinants of type 1, K88ab, K99 fimbriae and P(pap)pili of Escherichia coli is presented. The functions of the various gene products are described and a model for the process of fimbriae biogenesis is presented and discussed.

Primary structure and subcellular localization of two fimbrial subunit-like proteins involved in the biosynthesis of K99 fibrillae

Analysis of the nucleotide sequence of the distal part of the fan gene cluster encoding the proteins involved in the biosynthesis of the fibrillar adhesin, K99, revealed the presence of two structural genes, fanG and fanH. The amino acid sequence of the gene products (FanG and FanH) showed significant homology to the amino acid sequence of the fibrillar subunit protein (FanC). Introduction of a site-specific frameshift mutation in fanG or fanH resulted in a simultaneous decrease in fibrillae production and adhesive capacity. Analysis of subcellular fractions showed that, in contrast to the K99 fibrillar subunit (FanC), both the FanH and the FanG protein were loosely associated with the outer membrane, possibly on the periplasmic side, but were not components of the fimbriae themselves.

Modification, processing, and subcellular localization in Escherichia coli of the pCloDF13-encoded bacteriocin release protein fused to the mature portion of beta-lactamase

A fusion between the pCloDF13-derived bacteriocin release protein and beta-lactamase was constructed to investigate the subcellular localization and posttranslational modification of the bacteriocin release protein in Escherichia coli. The signal sequence and 25 of the 28 amino acid residues of the mature bacteriocin release protein were fused to the mature portion of beta-lactamase. The hybrid protein (Mr, 31,588) was expressed in minicells and whole cells and possessed full beta-lactamase activity. Immunoblotting of subcellular fractions revealed that the hybrid protein is present in both the cytoplasmic and outer membranes of E. coli. Radioactive labeling experiments in the presence or absence of globomycin showed that the hybrid protein is modified with a diglyceride and fatty acids and is processed by signal peptidase II, as is the murein lipoprotein. The results indicated that the pCloDF13-encoded bacteriocin release protein is a lipoprotein which is associated with both membranes of E. coli cells.

Uncoupling of synthesis and release of cloacin DF13 and its immunity protein by Escherichia coli

The synthesis of the bacteriocin cloacin DF13 and its release into the culture medium were genetically uncoupled by subcloning the gene encoding the bacteriocin release protein (BRP) from pCloDF13. The gene was cloned under the control of the IPTG-inducible lpp-lac promoter-operator system on the expression vector pINIIIA1, giving pJL1. A 4 kb DNA fragment of pJL1, containing the tandem lpp-lac promoter, the BRP gene and lacI (BRP cassette), was cloned into the pCloDF13 derivative plasmid pJN67, which encodes cloacin DF13 but not the release protein. Furthermore, the pCloDF13 immunity protein gene was subcloned downstream of the temperature-inducible PL promoter of the expression vector pPLc236, together with the BRP cassette. Growth, induction and excretion experiments with Escherichia coli cells harbouring the constructed plasmids revealed that: the BRP is the only pCloDF13-derived gene product responsible for the observed growth inhibition and apparent lysis of strongly induced cells. This growth inhibition and lysis can be prevented by Mg2+ ions added to the culture medium, and involves induction of phospholipase A activity. The expression of the BRP gene can be regulated by varying the IPTG concentration. A separately controlled and moderate induced BRP synthesis can be used to bring about the release of large amounts of cloacin DF13 under conditions that allow a strong induction of the bacteriocin and which do not result in lysis of cells. Preliminary results indicated that the BRP can stimulate the release of immunity protein in the absence of cloacin or cloacin fragments.

Effects of divalent cations and of phospholipase A activity on excretion of cloacin DF13 and lysis of host cells

Induction of cloacin DF13 synthesis in Escherichia coli harbouring plasmid CloDF13 results in the release of cloacin DF13, inhibition of growth and ultimately in lysis of the host cells. Expression of the pCloDF13-encoded protein H is essential for both the release of cloacin DF13 and the lysis of the cells. The divalent cations Mg2+ and Ca2+ interfered with the mitomycin C-induced protein H-dependent lysis, but hardly affected the release of cloacin DF13. Essentially all of the bacteriocin was released from the cells before a detectable degradation of the peptidoglycan occurred, independent of the presence of mitomycin C. Experiments with phospholipase A mutants revealed that activation of detergent-resistant phospholipase A was essential for the export of cloacin DF13 across the outer membrane and the lysis of induced cells. Transport of cloacin DF13 across the cytoplasmic membrane was mainly dependent on protein H. A revised model for the excretion of cloacin DF13 is presented.

Plasmid-determined cloacin DF13-susceptibility in Enterobacter cloacae and Klebsiella edwardsii; identification of the cloacin DF13/aerobactin outer membrane receptor proteins

Both Enterobacter cloacae H478 and Klebsiella edwardsii S15 were shown to harbour a relatively large conjugative plasmid that coded for cloacin DF13-susceptibility and the production and uptake of a hydroxamate iron chelator, most probably aerobactin. Protein-blotting experiments with antiserum raised against the purified cloacin DF13/aerobactin receptor protein from Escherichia coli (Co1V-K30) revealed that the corresponding outer membrane receptor proteins of Ent. cloacae H478 and K. edwardsii S15 had apparent mol wts of 85 000 and 76 000, respectively. E. coli transconjugants harbouring either the plasmid from Ent. cloacae H478 or K. edwardsii S15 expressed a cloacin DF13/aerobactin outer membrane receptor protein with a mol wt of 74 000. The receptor protein encoded by the Ent. cloacae and K. edwardsii plasmids were immunologically more related to each other than to the pCo1V-K30-encoded receptor protein.

Excretion of proteins by gram-negative bacteria: export of bacteriocins and fimbrial proteins by Escherichia coli

In gram-negative bacteria only few proteins are exported across both the cytoplasmic membrane and the outer membrane which forms an extra barrier for protein excretion. In this review we describe the mechanisms of production and export of two types of plasmid-encoded proteins in Escherichia coli. These proteins are the bacteriocin cloacin DF13 and the K88ab and K99 fimbrial subunits. Specific so-called helper proteins located at different positions in the cell envelope play an essential role in the export of these proteins. The genetic organization, subcellular location and functions of these helper proteins, as well as the effects of mutations and culture conditions on the export of the proteins are described. Models for the export mechanisms are presented and future application possibilities for engineering foreign protein excretion in E. coli with these export systems are discussed.

Subcloning of the cloacin DF13/aerobactin receptor protein and identification of a pColV-K30-determined polypeptide involved in ferric-aerobactin uptake

A plasmid containing a pColV-K30 fragment that encoded only for the cloacin DF13/aerobactin receptor protein was constructed. Escherichia coli cells harboring this plasmid were sensitive to cloacin DF13 but were unable to take up ferric-aerobactin. Another pColV-K30-determined polypeptide (molecular weight, 50,000), localized in the membrane fraction, was essential for the uptake of ferric-aerobactin.

Cloning and expression of the cloacin DF13/aerobactin receptor of Escherichia coli (ColV-K30)

A DNA fragment derived from the ColV-K30 plasmid and coding for both sensitivity to cloacin DF13 and Fe3+-aerobactin uptake was cloned into pBR322. The cloned fragment coded for two polypeptides with molecular masses of 74,000 (the cloacin DF13/aerobactin receptor protein) and 50,000 daltons, respectively. When grown with sufficient iron, cells harboring pFS8 (with this fragment) possessed about 10 times as many receptor protein molecules as compared with cells of Escherichia coli (ColV-K30). The synthesis of the receptor protein specified by pFS8, however, was independent of the availability of iron, in contrast to strains harboring the intact ColV-K30 plasmid. Aerobactin was taken up but not synthesized by cells harboring pFS8. No growth occurred when iron-starved cultures of these cells were incubated with Fe3+-aerobactin, suggesting that expression of other ColV-K30-encoded genes is necessary to remove the iron from the Fe3+-aerobactin complex.

Protein H encoded by plasmid CloDF13 is involved in excretion of cloacin DF13

Excretion of cloacin DF13 was studied in Escherichia coli cells harboring different CloDF13 insertion and deletion mutant plasmids. Insertions of a transposon at position 9.8 or 11.5% of the CloDF13 plasmid blocked the expression of gene H and strongly reduced the specific excretion of cloacin DF13 into the culture medium, but had no effect on the production of cloacin DF13. Insertions in or deletions of regions of the CloDF13 DNA upstream the cloacin operon did not affect the excretion or production of the bacteriocin. Introduction of a CloDF13 plasmid that encodes for the gene H product in cells harboring a CloDF13 plasmid with an insertion in gene H stimulated the excretion of cloacin DF13 significantly in mitomycin C-induced and in noninduced cultures. Cloacin DF13 in cloacinogenic cells that did not produce the gene H protein was found to be about 90% located in the cytoplasm. In cells that did produce the gene H product, about 30% of the cloacin DF13 molecules were found in the cytoplasm, about 18% were found in the periplasm, about 2% were in the membranes, and about 50% were located in the culture supernatant. Cyclic AMP stimulated the production but not the excretion of cloacin DF13 in cells cultivated in the presence of glucose.

Siderophore production by Enterobacter cloacae and a common receptor protein for the uptake of aerobactin and cloacin DF13

Iron-starved cultures of Enterobacter cloacae produced two siderophores, identified as enterochelin and aerobactin. The aerobactin was excreted in larger amounts than was enterochelin, and it was synthesized preferentially in the late logarithmic and stationary growth phases under iron-deficient conditions. Enterochelin was synthesized by cultures in the logarithmic phase of growth and preferentially in medium with 1 microM ferric chloride. Both siderophores appeared to be excreted immediately after their synthesis, since no intracellular aerobactin or enterochelin could be detected. The killing activity of the bacteriocin cloacin DF13 was inhibited by aerobactin. It was shown that aerobactin and cloacin DF13 bound to the same receptor sites located in the outer membrane. The synthesis of these receptor sites was induced by iron limitation. We conclude that the receptor for the uptake of aerobactin also functions as receptor for cloacin DF13.

In vitro binding of cloacin DF13 to its purified outer membrane receptor protein and effect of peptidoglycan on bacteriocin-receptor interaction

The in vitro neutralization of the killing activity of cloacin DF13 by incubation with its purified receptor protein was shown to be the result of the formation of a direct and specific equimolar complex of both proteins. The binding of cloacin DF13 to its receptor protein did not result in a fragmentation of the cloacin molecules nor in the expulsion of immunity protein from the bacteriocin. The rate of the cloacin DF13-receptor interaction in vitro was found to be enhanced significantly in the presence of peptidoglycan, but lysozyme-treated peptidoglycan did not affect this interaction. Incubation of the cloacin DF13 as well as its receptor protein with peptidoglycan showed that the receptor protein but not the cloacin DF13 was able to bind to the peptidoglycan.

Purification and characterization of cloacin DF13 receptor from Enterobacter cloacae and its interaction with cloacin DF13 in vitro

Extraction of the crude cell envelope fraction of cloacin DF13-susceptible Enterobacter cloacae strain 02 with Triton X-100 and ethylenediaminetetraacetate solubilized an outer membrane fraction which neutralized the lethal activity of cloacin DF13. A similar fraction could not be isolated from strains known to be lacking functional cloacin DF13 receptors. On this basis the isolated outer membrane fraction was assumed to contain the specific cloacin DF13 receptor. The receptor was purified to homogeneity by acetone precipitation and affinity chromatography, using cloacin DF13 as a ligand. The purified receptor was identified as a protein which consisted of a single polypeptide chain with an apparent molecular weight of 90,000 and a preponderance of acidic amino acids (pI = 5.0). The interaction of equimolar amounts of purified receptor and cloacin DF13 in vitro resulted in a complete, irreversible neutralization of the lethal activity of the bacteriocin. This interaction showed a temperature optimum at 43 degrees C but was only slightly affected by variation of the pH between 5.0 and 8.5 or by increasing the ionic strength of the incubation buffer. The receptor had no neutralizing activity towards other bacteriocins, such as colicin E1 or colicin E3.

The use of mutants in the study of structure-function relationships in cloacin DF13

A bacteriocin from cells with a mutant Clo DF13 plasmid (cloacin clp03 . immunity protein complex) and a bacteriocin from cells containing the recombinant plasmic Clo DF13 :: Tn901 (cloacin pJN82) have been isolated. Both bacteriocins like wild-type cloacin DF13, are still able to inhibit in vitro protein synthesis, but their in vivo killing activity is absent. Comparison of some physicochemical characteristics of the cloacin clp03 . immunity protein complex and wild-type cloacin complex showed no significant differences. From a comparison of the binding capacity to specific receptors on sensitive cells, the translocation through the cell wall, and the interaction with cytoplasmic membranes, it could be concluded that the cloacin clp03 complex is hampered in its translocation from the outer membrane receptor site to the cytoplasmic membrane, resulting in the observed lack in killing activity. Cloacin pJN82 is shortened at the C-terminal of the molecule by approximately ten amino acid residues. Together with its loss of in vivo killing activity it has lost its capacity to bind immunity protein. Since the immunity protein probably not only provides cloacin-producing cells with "immunity" but is also involved in the translocation of the bacteriocin to the interior of sensitive cells, the absence of this protein is probably the reason for the lack of killing activity of cloacin pJN82. The implications of these findings for the topography of the cloacin molecule as suggested by de Graaf et al. (de Graaf, F.K., Stukart, M.J., Boogerd, F.C. and Metselaar, K. (1978) Biochemistry, in press) are discussed.

Enzymatic properties of cloacin DF13 and kinetics of ribosome inactivation

1. The cloacin DF13-induced inactivation of ribosomes in vitro can be described as an enzyme-catalyzed reaction according to the Michaelis-Menten equation. Most probably the cloacin acts as a unique endoribonuclease. 2. At pH 7.8 and 37 degrees C the Km value for the reaction of cloacin DF13 with ribosomes is 13.2 - 10(-6) M. If under these conditions the reaction mixture is supplemented with all components necessary for protein synthesis, the Km changes to 17.7 - 10(-6) M. 3. The in vitro activity of cloacin DF13 has a temperature optimum of 43 degrees C at pH 7.8 and a pH optimum of 8.4 at 37 degtees C. 4. Experiments with cloacin DF13-immunity protein as an inhibitor of the cloacin activity in vitro have indicated that the immunity protein might be considered as a non-competitive and virtually "irreversible" inhibitor.

Effects of temperature on the activity of cloacin DF13 and cloacin DF13-immunity protein

During the interaction of cloacin DF13 and sensitive cells the cloacin molecules display different functions which can be distinguished on the basis of their heat-sensitivity. Binding to cell envelope receptors, binding of immunity protein and in vitro inactivation of ribosomes are heat-stable functions in contrast with the entire killing action in vivo. Cloacin DF13-immunity protein appears to be a heat-stable inhibitor of the fibosome inactivation caused by cloacin DF13.

Mode of action of the cloacin DF13-immunity protein

1. Cells of Enterobacter cloacae harbouring the bacteriocinogenic factor Clo DF13 produce an immunity protein which inbhbits the in vitro activity of cloacin DF13. The amino acid composition of purified immunity protein was determined. 2. Experiments about the protection of ribosomes against cloacin DF13 in the presence of the immunity protein show that one molecule of immunity protein neutralized the activity of one molecule cloacin. 3. Direct and specific interaction of cloacin DF13 with the immunity protein has been demonstrated by the analysis of mixtures of both proteins on polyacrylamide gels and by changes in the fluorescence response of cloacin DF13-bound 1-anilinonaphthalene-8-sulfonate in the presence of immunity protein.