Molecular and genetic analysis of a region of plasmid pCF10 containing positive control genes and structural genes encoding surface proteins involved in pheromone-inducible conjugation in Enterococcus faecalis

Exposure of Enterococcus faecalis cells carrying the tetracycline resistance plasmid pCF10 to the heptapeptide pheromone cCF10 results in an increase in conjugal transfer frequency by as much as 10(6)-fold. Pheromone-induced donor cells also express at least two plasmid-encoded surface proteins, the 130-kDa Sec 10 protein, which is involved in surface exclusion, and the 150-kDa Asc10 protein, which has been associated with the formation of mating aggregates. Previous subcloning and transposon mutagenesis studies indicated that the adjacent EcoRI c (7.5 kb) and e (4.5 kb) fragments of pCF10 encode the structural genes for these proteins and that the EcoRI c fragment also encodes at least two regulatory genes involved in activation of the expression of the genes encoding Asc10 and Sec10. In this paper, the results of physical and genetic analysis of this region of pCF10, along with the complete DNA sequences of the EcoRI c and e fragments, are reported. The results of the genetic studies indicate the location of the structural genes for the surface proteins and reveal important features of their transcription. In addition, we provide evidence here and in the accompanying paper (S. B. Olmsted, S.-M. Kao, L. J. van Putte, J. C. Gallo, and G. M. Dunny, J. Bacteriol. 173:7665-7672, 1991) for a role of Asc10 in mating aggregate formation. The data also reveal a complex positive control system that acts at distances of at least 3 to 6 kb to activate expression of Asc10. DNA sequence analysis presented here reveals the positions of a number of specific genes, termed prg (pheromone-responsive genes) in this region of pCF10. The genes mapped include prgA (encoding Sec10) and prgB (encoding Asc10), as well as four putative regulatory genes, prgX, -R, -S, and -T. Although the predicted amino acid sequences of Sec10 and Asc10 have some structural features in common with a number of surface proteins of gram-positive cocci, and the Asc10 sequence is highly similar to that of a similar protein encoded by the pheromone-inducible plasmid pAD1 (D. Galli, F. Lottspeich, and R. Wirth, Mol. Microbiol. 4:895-904, 1990), the regulatory genes show relatively little resemblance to any previously sequenced genes from either procaryotes or eucaryotes.

Structural analyses of a channel-forming fragment of colicin E1 incorporated into lipid vesicles. Fourier-transform infrared and tryptophan fluorescence studies

Structural changes upon binding to the membrane of a COOH-terminal channel-forming thermolytic fragment of colicin E1 have been studied by means of a variety of spectroscopic techniques. Circular dichroism measurements show that the thermolytic fragment predominantly takes a helical structure in aqueous and detergent solutions. Fourier transform infrared spectroscopic measurements indicate that the content of the beta-structure is significantly increased when the thermolytic fragment is bound to vesicles. On the basis of the result of tryptophan fluorescence measurements, we have concluded that each of the three tryptophan residues of the thermolytic fragment exists in different environments, i.e. one is buried in the lipid bilayer, one exists on the cis side of the vesicles, and one exists near the surface of the lipid bilayer. The Fourier transform infrared and fluorescence data have been used along with the crystal structure of colicin A, which is highly homologous to colicin E1 in structure and function, to propose a model of the thermolytic fragment bound to the lipid vesicles.

Membrane topography of ColE1 gene products: the hydrophobic anchor of the colicin E1 channel is a helical hairpin

The paucity of crystallographic data on the structure of intrinsic membrane proteins necessitates the development of additional techniques to probe their structures. The colicin E1 ion channel domain contains one prominent hydrophobic region near its COOH terminus that has been proposed to be an anchor for the assembly of the channel. Saturation site-directed mutagenesis of the hydrophobic anchor region of the colicin E1 ion channel was used to probe whether it spanned the bilayer once or twice. A nonpolar amino acid was replaced by a charged residue in 29 mutations made at 26 positions in the channel domain. Substitution of the charged amino acid at all positions except those in the center of the hydrophobic region and the periphery of the hydrophobic region caused a large decrease in the cytotoxicity of the purified mutant colicin E1 protein. This result implies that the hydrophobic domain spans the membrane bilayer twice in a helical hairpin loop, with the center of this domain residing in an aqueous or polar phase. The lengths of the trans-membrane helices appear to be approximately 18 and 16 residues. The absence of significant changes in ion selectivity in five of nine mutants indicated that these mutations did not cause a large change in the channel structure. The ion selectivity changes in four mutants and those previously documented for the flanking Lys residues imply that the hydrophobic hairpin is part of the channel lumen. Water may "abhor" the hydrophobic side of the channel, explaining the small effects of residue charge changes on ion selectivity.

Fluorescence energy transfer distance measurements using site-directed single cysteine mutants. The membrane insertion of colicin A

The ion-channel-forming C-terminal fragment of colicin A binds to negatively charged lipid vesicles and provides an example of insertion of a soluble protein into a lipid bilayer. The soluble structure is known from X-ray crystallography and consists of a ten-helix bundle containing a hydrophobic helical hairpin. In this work fluorescence spectroscopy was used to study the membrane-bound structure. An extrinsic probe, N'-(iodoacetyl)-N'-(5-sulfol-naphthyl)ethylenediamine (IAEDANS) was attached to mutant proteins each of which bears a unique cysteine residue. Three mutants K39C (helix 2), T127C (between helices 6 and 7) and S16Crpt (helix 1, which bears a decapeptide repeat before the mutation) gave useful derivatives. In the soluble protein they showed emission wavelengths decreasing in the order K39C greater than T127C greater than S16Crpt and although all showed blue shifts on addition of dimyristoylphosphatidylglycerol (DMPG) this order was maintained in the membrane-bound state. These shifts were not indicative of deep membrane insertion. Polarization of IAEDANS revealed differences in mobility between mutants. The three tryptophan residues were used as a compound donor to IAEDANS in resonance energy transfer distance determinations. The values obtained for the soluble form were 1.2 A to 3.2 A longer than in the crystal structure. On addition of lipids the indicated distances increased: S16Crpt-I(AEDANS) 6.45 A (22%), K39C-I 5.45 A (18%) and T127C-I 2.4 A (14%). N-bromosuccinimide (NBS) completely abolishes the tryptophan emission from the thermolytic fragment. When lipids were added to a mixture containing ten NBS-treated channel-forming fragments to one IAEDANS labelled fragment the indicated distances increased rather more: S16Crpt-I 9.7 A (38%), K39C-I 8.1 A (36%) and T127C-I 2.5 A (16%). This showed that intermolecular transfer reduces the distance estimated in samples containing only labelled protein. The ensemble of results shows that the amphipathic helices of the C-terminal fragment open out on the surface of the lipid bilayer during the initial phase of membrane insertion.

Membrane insertion of the pore-forming domain of colicin A. A spectroscopic study

In order to gain some insight into the mechanism of insertion into membranes of the pore-forming domain of colicin A and the structure of its membrane-bound form, circular dichroism (in the near and far ultraviolet), fluorescence and ultraviolet spectroscopy experiments were carried out. Because the structure of the water-soluble form of this fragment has been determined by X-ray crystallography, these spectroscopic methods provided valuable information on the secondary structure and the environment of aromatic residues within the two forms of the peptide. These results strongly suggest that the pore-forming domain of colicin A does not undergo drastic unfolding upon insertion into membrane. The conformational change associated with this process is triggered by the negatively charged lipids and probably consists of a reorientation of helix pairs with respect to each other. Exposure of the aromatic residues to the aqueous phase decreases on binding to lipids whilst the exposure of the tryptophans to the membrane phase increases. This cannot occur without a reorientation of helices 3-10. All data from this study support the model presented previously in which the known crystal structure opens like an 'umbrella' inserting the hydrophobic hairpin (helix 8-9) perpendicular to the membrane plane and the helical pair 1-2 and the domain containing the three tryptophans (helices 3-7) lying more or less parallel to the membrane plane. Lipids are bound more tightly to the protein at acidic pH than at neutral pH although a similar lipid protein complex is formed with 1,2-dimyristoyl-sn-glycero(3)-phospho(1)- -sn-glycerol at both pH values.

Fourier transform infrared evidence for a predominantly alpha-helical structure of the membrane bound channel forming COOH-terminal peptide of colicin E1

The structure of the membrane bound state of the 178-residue thermolytic COOH-terminal channel forming peptide of colicin E1 was studied by polarized Fourier transform infrared (FTIR) spectroscopy. This fragment was reconstituted into DMPC liposomes at varying peptide/lipid ratios ranging from 1/25-1/500. The amide I band frequency of the protein indicated a dominant alpha-helical secondary structure with limited beta- and random structures. The amide I and II frequencies are at 1,656 and 1,546 cm-1, close to the frequency of the amide I and II bands of rhodopsin, bacteriorhodopsin and other alpha-helical proteins. Polarized FTIR of oriented membranes revealed that the alpha-helices have an average orientation less than the magic angle, 54.6 degrees, relative to the membrane normal. Almost all of the peptide groups in the membrane-bound channel protein undergo rapid hydrogen/deuterium (H/D) exchange. These results are contrasted to the alpha-helical membrane proteins, bacteriorhodopsin, and rhodopsin.

Construction, expression and release of hybrid colicins

Colicins A and E1 are two pore-forming colicins sharing homology in their C-terminal domains but not in their N-terminal or central domains. Using site-directed mutagenesis, restriction sites were inserted at the proper locations to allow recombination of these domains. Six different constructs were obtained. All these proteins were expressed in Escherichia coli and properly recognized by monoclonal antibodies directed against epitopes located in different domains of colicin A. Out of the six hybrids, only two were released to the extracellular medium. Immunocytolocalization indicated that some of the hybrids aggregated within the cytoplasm. With some hybrids, the defect in release was related to a defect in synthesis of the lysis protein that normally promotes release.

Individual domains of colicins confer specificity in colicin uptake, in pore-properties and in immunity requirement

Six different hybrid colicins were constructed by recombining various domains of the two pore-forming colicins A and E1. These hybrid colicins were purified and their properties were studied. All of them were active against sensitive cells, although to varying degrees. From the results, one can conclude that: (1) the binding site of OmpF is located in the N-terminal domain of colicin A; (2) the OmpF, TolB and TolR dependence for translocation is also located in this domain; (3) the TolC dependence for colicin E1 is located in the N-terminal domain of colicin E1; (4) the 183 N-terminal amino acid residues of colicin E1 are sufficient to promote E1AA uptake and thus probably colicin E1 uptake; (5) there is an interaction between the central domain and C-terminal domain of colicin A; (6) the individual functioning of different domains in various hybrids suggests that domain interactions can be reconstituted in hybrids that are fully active, whereas in others that are much less active, non-proper domain interactions may interfere with translocation; (7) there is a specific recognition of the C-terminal domains of colicin A and colicin E1 by their respective immunity proteins.

Structure of alpha-alpha-hairpins with short connections

This paper presents the results of detailed stereochemical analysis of structures and sequences of alpha-alpha-hairpins with short connections. It is shown that alpha-alpha-hairpins of each given type have very similar patterns of hydrophobic, hydrophilic and glycine residues in their amino acid sequences. These results can be used in the prediction of alpha-alpha-hairpin conformation as well as in protein design and engineering.

Colicinogeny of O157:H7 enterohemorrhagic Escherichia coli and the shielding of colicin and phage receptors by their O-antigenic side chains

Twenty O157:H7 enterohemorrhagic Escherichia coli strains from patients with different clinical conditions were tested for colicinogeny and the presence of Verotoxin (VT) genes. From bloody diarrhea cases, 7/8 isolates and from hemolytic uremic syndrome cases 3/5 isolates all synthesized what appeared to be colicin D. The remaining strains, which included 7 from asymptomatic sources, were noncolicinogenic. The plasmid determining the colicin was found to be 1.4 kb larger than the 5.2-kb pColD. The colicin D protein had a molecular weight of about 90,000, whereas the O157 colicin was 87,000. The plasmid was designated pColD157 to reflect these differences. Of O157:H7 isolates 17/20 had genes for both of the Verotoxins VT1 and VT2, and the remaining 3/20 for VT1 only. There was no correlation between the presence of VT determinants and colicinogeny or symptoms. The O157:H7 strains exhibited significant resistance to other colicins and bacteriophages.