Interaction between the poly(A)-binding protein Pab1 and the eukaryotic release factor eRF3 regulates translation termination but not mRNA decay in Saccharomyces cerevisiae.

Eukaryotic release factor 3 (eRF3) is implicated in translation termination and also interacts with the poly(A)-binding protein (PABP, Pab1 in yeast), a major player in mRNA metabolism. Despite conservation of this interaction, its precise function remains elusive. First, we showed experimentally that yeast eRF3 does not contain any obvious consensus PAM2 (PABP-interacting motif 2). Thus, in yeast this association is different from the well described interaction between the metazoan factors. To gain insight into the exact function of this interaction, we then analyzed the phenotypes resulting from deleting the respective binding domains. Deletion of the Pab1 interaction domain on eRF3 did not affect general mRNA stability or nonsense-mediated mRNA decay (NMD) pathway and induced a decrease in translational readthrough. Furthermore, combined deletions of the respective interacting domains on eRF3 and on Pab1 were viable, did not affect Pab1 function in mRNA stability and harbored an antisuppression phenotype. Our results show that in Saccharomyces cerevisiae the role of the Pab1 C-terminal domain in mRNA stability is independent of eRF3 and the association of these two factors negatively regulates translation termination.

The Stable Interaction Between Signal Peptidase LepB of Escherichia coli and Nuclease Bacteriocins Promotes Toxin Entry into the Cytoplasm.

LepB is a key membrane component of the cellular secretion machinery, which releases secreted proteins into the periplasm by cleaving the inner membrane-bound leader. We showed that LepB is also an essential component of the machinery hijacked by the tRNase colicin D for its import. Here we demonstrate that this non-catalytic activity of LepB is to promote the association of the central domain of colicin D with the inner membrane before the FtsH-dependent proteolytic processing and translocation of the toxic tRNase domain into the cytoplasm. The novel structural role of LepB results in a stable interaction with colicin D, with a stoichiometry of 1:1 and a nanomolar Kd determined in vitro. LepB provides a chaperone-like function for the penetration of several nuclease-type bacteriocins into target cells. The colicin-LepB interaction is shown to require only a short peptide sequence within the central domain of these bacteriocins and to involve residues present in the short C-terminal Box E of LepB. Genomic screening identified the conserved LepB binding motif in colicin-like ORFs from 13 additional bacterial species. These findings establish a new paradigm for the functional adaptability of an essential inner-membrane enzyme.

Mechanism of activation of methyltransferases involved in translation by the Trm112 'hub' protein.

Methylation is a common modification encountered in DNA, RNA and proteins. It plays a central role in gene expression, protein function and mRNA translation. Prokaryotic and eukaryotic class I translation termination factors are methylated on the glutamine of the essential and universally conserved GGQ motif, in line with an important cellular role. In eukaryotes, this modification is performed by the Mtq2-Trm112 holoenzyme. Trm112 activates not only the Mtq2 catalytic subunit but also two other tRNA methyltransferases (Trm9 and Trm11). To understand the molecular mechanisms underlying methyltransferase activation by Trm112, we have determined the 3D structure of the Mtq2-Trm112 complex and mapped its active site. Using site-directed mutagenesis and in vivo functional experiments, we show that this structure can also serve as a model for the Trm9-Trm112 complex, supporting our hypothesis that Trm112 uses a common strategy to activate these three methyltransferases.

Molecular and Clinical Data of Antimicrobial Resistance in Microorganisms Producing Bacteremia in a Multicentric Cohort of Patients with Cancer in a Latin American Country.

Patients with cancer have a higher risk of severe bacterial infections. This study aims to determine the frequency, susceptibility profiles, and resistance genes of bacterial species involved in bacteremia, as well as risk factors associated with mortality in cancer patients in Colombia. In this prospective multicenter cohort study of adult patients with cancer and bacteremia, susceptibility testing was performed and selected resistance genes were identified. A multivariate regression analysis was carried out for the identification of risk factors for mortality. In 195 patients, 206 microorganisms were isolated. Gram-negative bacteria were more frequently found, in 142 cases (68.9%): 67 Escherichia coli (32.5%), 36 Klebsiella pneumoniae (17.4%), and 21 Pseudomonas aeruginosa (10.1%), and 18 other Gram-negative isolates (8.7%). Staphylococcus aureus represented 12.4% (n = 25). Among the isolates, resistance to at least one antibiotic was identified in 63% of them. Genes coding for extended-spectrum beta-lactamases and carbapenemases, blaCTX-M and blaKPC, respectively, were commonly found. Mortality rate was 25.6% and it was lower in those with adequate empirical antibiotic treatment (22.0% vs. 45.2%, OR: 0.26, 95% CI: 0.1-0.63, in the multivariate model). In Colombia, in patients with cancer and bacteremia, bacteria have a high resistance profile to beta-lactams, with a high incidence of extended-spectrum beta-lactamases and carbapenemases. Adequate empirical treatment diminishes mortality, and empirical selection of treatment in this environment of high resistance is of key importance.

FtsH-dependent processing of RNase colicins D and E3 means that only the cytotoxic domains are imported into the cytoplasm.

It has long been suggested that the import of nuclease colicins requires protein processing; however it had never been formally demonstrated. Here we show that two RNase colicins, E3 and D, which appropriate two different translocation machineries to cross the outer membrane (BtuB/Tol and FepA/TonB, respectively), undergo a processing step inside the cell that is essential to their killing action. We have detected the presence of the C-terminal catalytic domains of these colicins in the cytoplasm of target bacteria. The same processed forms were identified in both colicin-sensitive cells and in cells immune to colicin because of the expression of the cognate immunity protein. We demonstrate that the inner membrane protease FtsH is necessary for the processing of colicins D and E3 during their import. We also show that the signal peptidase LepB interacts directly with the central domain of colicin D in vitro and that it is a specific but not a catalytic requirement for in vivo processing of colicin D. The interaction of colicin D with LepB may ensure a stable association with the inner membrane that in turn allows the colicin recognition by FtsH. We have also shown that the outer membrane protease OmpT is responsible for alternative and distinct endoproteolytic cleavages of colicins D and E3 in vitro, presumably reflecting its known role in the bacterial defense against antimicrobial peptides. Even though the OmpT-catalyzed in vitro cleavage also liberates the catalytic domain from colicins D and E3, it is not involved in the processing of nuclease colicins during their import into the cytoplasm.

Hijacking cellular functions for processing and delivery of colicins E3 and D into the cytoplasm.

The mechanisms for importing colicins from the extracellular medium into Escherichia coli target cells implicate a complex cascade of interactions with host proteins. It is known that colicins interact with membrane receptors, and they may appropriate them structurally, but not functionally, as a scaffold on the surface of the target cell so that they can be translocated across the outer membrane. During the import into the periplasm, colicins parasitize functionally membrane porins and energy-transducers by mimicking their natural substrates or interacting partners. Such structural or functional parasitism also takes place during the late molecular events responsible for the processing and translocation of nuclease colicins across the inner membrane. Two different RNase colicins (D and E3) require an endoproteolytic cleavage, dependent on the inner membrane ATPase/protease FtsH, in order to transfer their C-terminal toxic domain into the cytoplasm. Moreover, the processing of colicin D necessitates a specific interaction with the signal peptidase LepB, but without appropriating the catalytic activity of this enzyme. A comparison of the differences in structural and functional organizations of these two colicins, as well as the pore-forming colicin B, is discussed in the present paper in connection with the sequential steps of their import mechanisms and the exploitation of the machinery of the target cell.

Prevalence and Outcome of Serum Autoantibodies in Chronic Hepatitis C Patients Undergoing Direct-Acting Antiviral Treatment.

Background: Chronic immune stimulation by hepatitis C virus (HCV) may cause occurrence of several autoantibodies in infected patients, with or without features of clinically overt autoimmune diseases. The recent introduction of direct-acting antivirals (DAAs) has dramatically changed the natural history of chronic HCV infection. The aim of this study was to assess the effects of DAA therapy on serum autoantibodies in chronic hepatitis C (CHC) patients. Methods: The medical records of 113 CHC patients were reviewed to assess autoantibody behavior following DAA-directed HCV eradication. Statistical analysis was performed to assess correlations between DAA treatment and autoantibody titers, HCV genotypes, and viral loads. Results: Anti-nuclear (ANA), anti-smooth muscle cell (ASMA) and anti-mitochondrial (AMA) antibody testing was available in 77 patients; 31 out of 77 patients (40%) had one or more serum autoantibodies prior to treatment. Measurement of autoantibody titers before and after HCV eradication was performed in 20 of 31 patients. DAA treatment significantly affected ANA and ASMA titers, leading to disappearance or reduction of autoantibody titers; conversely, AMA were not influenced by DAA treatment. No correlations were observed between autoantibody specificity and both HCV genotypes and viral loads at baseline. Likewise, serum autoantibody titers were independent of HCV genotypes. Conclusions: DAA-directed HCV clearance may interrupt chronic immune stimulation by removing the drive for autoantibody induction. The isolated persistence of autoantibodies in the small fraction of patients who did not show clearance following DAA treatment may require long-term vigilance.

Novel Escherichia coli RF1 mutants with decreased translation termination activity and increased sensitivity to the cytotoxic effect of the bacterial toxins Kid and RelE.

Novel mutations in prfA, the gene for the polypeptide release factor RF1 of Escherichia coli, were isolated using a positive genetic screen based on the parD (kis, kid) toxin-antitoxin system. This original approach allowed the direct selection of mutants with altered translational termination efficiency at UAG codons. The isolated prfA mutants displayed a approximately 10-fold decrease in UAG termination efficiency with no significant changes in RF1 stability in vivo. All three mutations, G121S, G301S and R303H, were situated close to the nonsense codon recognition site in RF1:ribosome complexes. The prfA mutants displayed increased sensitivity to the RelE toxin encoded by the relBE system of E. coli, thus providing in vivo support for the functional interaction between RF1 and RelE. The prfA mutants also showed increased sensitivity to the Kid toxin. Since this toxin can cleave RNA in a ribosome-independent manner, this result was not anticipated and provided first evidence for the involvement of RF1 in the pathway of Kid toxicity. The sensitivity of the prfA mutants to RelE and Kid was restored to normal levels upon overproduction of the wild-type RF1 protein. We discuss these results and their utility for the design of novel antibacterial strategies in the light of the recently reported structure of ribosome-bound RF1.

In Vitro Study of the Major Bacillus subtilis Ribonucleases Y and J.

The metabolic instability of mRNA is fundamental to the adaptation of gene expression. In bacteria, mRNA decay follows first-order kinetics and is primarily controlled at the steps initiating degradation. In the model Gram-positive organism Bacillus subtilis, the major mRNA decay pathway initiates with an endonucleolytic cleavage by the membrane-associated RNase Y. High-throughput sequencing has identified a large number of potential mRNA substrates but our understanding of what parameters affect cleavage in vivo is still quite limited. In vitro reconstitution of the cleavage event is thus instrumental in defining the mechanistic details, substrate recognition, the role of auxiliary factors, and of membrane localization in cleavage. In this chapter, we describe not only the purification and assay of RNase Y but also RNase J1/J2 which shares a similar low-specificity endoribonucleolytic activity with RNase Y. We highlight potential problems in the set-up of these assays and include methods that allow purification of full-length RNase Y and its incorporation in multilamellar vesicles created from native B. subtilis lipids that might best mimic in vivo conditions.

Vigorous exercise acutely changes platelet and B-lymphocyte CD39 expression.

CD39/ATP diphosphohydrolase is expressed on B lymphocytes, cytotoxic T lymphocytes, monocytes, platelets, and endothelial cells, and it has a critical role in the inhibition of platelet responsiveness. To determine whether strenuous exercise could acutely change expression of CD39 in platelets and lymphocytes, eight healthy sedentary men, 34 yr old (SD 7), and eight physically active men, 34 yr old (SD 6), performed graded upright cycle ergometry to volitional exhaustion. Blood samples collected both at baseline and after exercise test were employed to measure CD39 expression in platelets and lymphocytes. The percentage of circulating platelet-platelet aggregates, the "in vitro" ADP and collagen-induced platelet aggregation, and the expression of both platelet glycoprotein IIb-IIIa (PAC-1) and P-selectin (CD62) were also considered markers of platelet activation. After strenuous exercise, all subjects demonstrated significant platelet activation as judged by the increased percentage of platelet-platelet aggregates. The in vitro ADP-induced platelet aggregation and the expression of CD62P on ADP-stimulated platelets significantly increased in sedentary but not in active subjects. After exercise, all of the subjects showed a significant reduction of CD39 expression in platelet [sedentary: from 2.2 (SD 0.8) to 1.1% (SD 0.8), P = 0.008; active: from 0.6 (SD 0.2) to 0.35% (SD 0.1), P = 0.009] and an increase of CD39 expression in B lymphocytes [sedentary: from 47 (SD 13) to 60% (SD 11), P = 0.0039; active: from 46 (SD 11) to 59% (SD 11), P = 0.0038]. Taken together, these findings confirm the critical role of this ADPase in inhibition of platelet responsiveness, also suggesting a possible role of B lymphocytes in thromboregulation mechanism.

Effects of preactivated autologous T lymphocytes on CD80, CD86 and CD95 expression by chronic lymphocytic leukemia B cells.

Profound immune dysfunction is a constant feature in B-cell chronic lymphocytic leukemia (B-CLL) patients. Immunological abnormalities include hypogammaglobulinemia, impaired immunoglobulin class switching and diminished germinal center formation. This state of immune suppression renders B-CLL patients highly susceptible to infections, which contribute greatly to morbidity and mortality in this disease. Impaired T cell function in B-CLL is well-documented and has been suggested to result from inhibitory effects exerted by malignant B lymphocytes. Because the presence of leukemic cells may represent a major obstacle to efficient T cell activation, T lymphocytes were separated from CLL B cells, stimulated with phorbol 12-myristate 13-acetate (PMA) and ionomycin for 4h, and then cocultured with autologous leukemic B cells both at a 1:1 ratio or at the same ratio as in vivo for 24-40 h. CLL B cell expression of CD86 and CD95 was markedly upregulated using this approach, whereas CD80 expression was augmented only in a minority of patients; these effects were partially preserved even when preactivated T cells were rechallenged with CLL B cells at the same low T/B cell ratio as that observed in vivo. Finally, CD80 upregulation on CLL B cells appeared to be mainly dependent on CD40L-mediated stimulation, whereas CD86 and CD95 expression was efficiently augmented by soluble factors released by preactivated T lymphocytes. In conclusion, efficient activation of T lymphocytes in B-CLL may be achieved which, in turn, may result in enhanced antigen-presenting capacity and susceptibility to apoptosis of leukemic cells via CD86 and CD95 upregulation, respectively.

Differences in constitutive and activation-induced expression of CD69 and CD95 between normal and chronic lymphocytic leukemia B cells.

B-cell chronic lymphocytic leukemia (B-CLL) is characterized by a sustained accumulation of long-lived and well-differentiated B lymphocytes in lymphoid tissues, peripheral blood and bone marrow. Although the pathogenesis of this disease is not entirely understood, altered apoptosis is believed to play a relevant role in B-CLL. In this study, we compared the expression of CD95, the best characterized surface molecule involved in triggering the apoptotic machinery, on normal and CLL B cells before and after in vitro activation with polyclonal stimulators. Cell activation was monitored by verifying the induced expression of the early activation antigen CD69. Freshly analyzed CLL B cells showed significantly lower levels of CD95 than normal B cells. Moreover, following in vitro culture with phorbol 12-myristate 13-acetate (PMA) + ionomycin, phytohemagglutinin, or pokeweed mitogen, CLL B cells failed to upregulate CD95 expression as efficiently as normal B cells. Impairment of CD95 upregulation was mainly observed following PMA + ionomycin treatment. In contrast, CLL B cells were shown to express CD69 as well as normal B cells, regardless of the activator used, indicating that CLL B cells retain the ability to respond to activating stimuli but are unable to efficiently implement the CD95-mediated activation-induced cell death (AICD) program. In conclusion, these results suggest that prolonged survival of CLL B cells may be contributed to by alterations in AICD mechanisms.

In vivo processing of DNase colicins E2 and E7 is required for their import into the cytoplasm of target cells.

DNase colicins E2 and E7, both of which appropriate the BtuB/Tol translocation machinery to cross the outer membrane, undergo a processing step as they enter the cytoplasm. This endoproteolytic cleavage is essential for their killing action. A processed form of the same size, 18.5 kDa, which corresponds to the C-terminal catalytic domain, was detected in the cytoplasm of bacteria treated with either of the two DNase colicins. The inner-membrane protease FtsH is necessary for the processing that allows the translocation of the colicin DNase domain into the cytoplasm. The processing occurs near residue D420, at the same position as the FtsH-dependent cleavage in RNase colicins E3 and D. The cleavage site is located 30 amino acids upstream of the DNase domain. In contrast, the previously reported periplasm-dependent colicin cleavage, located at R452 in colicin E2, was shown to be generated by the outer-membrane protease OmpT and we show that this cleavage is not physiologically relevant for colicin import. Residue R452, whose mutated derivatives led to toxicity defect, was shown to have no role in colicin processing and translocation, but it plays a key role in the catalytic activity, as previously reported for other DNase colicins. Membrane associated forms of colicins E2 and E7 were detected on target cells as proteinase K resistant peptides, which include both the receptor-binding and DNase domains. A similar, but much less proteinase K-resistant form was also detected with RNase colicin E3. These colicin forms are not relevant for colicin import, but their detection on the cell surface indicates that whole nuclease-colicin molecules are found in a stable association with the outer-membrane receptor BtuB of the target cells.

Epstein Barr virus infection reactivation as a possible trigger of primary biliary cirrhosis-like syndrome in a patient with multiple sclerosis in the course of fingolimod treatment.

Primary biliary cirrhosis (PBC) and multiple sclerosis (MS) are considered autoimmune diseases with a multifactorial aetiology which is thought to be due to a combination of genetic predisposition and environmental triggers. An association of both diseases has been previously described in sporadic case reports. Fingolimod, an antagonist of the sphingosine 1 phosphate receptor family (S1P1/3/4/5), is a promising and effective drug in the treatment of MS. Here we describe a case of PBC like syndrome that was unmasked, concomitantly or consequently to Epstein Barr virus (EBV) infection reactivation, in a 34 year old male patient with relapsing remitting multiple sclerosis who was receiving fingolimod treatment.

Rituximab: eficacia, efectividad y seguridad en el tratamiento de la esclerosis múltiple / Rituximab: its efficacy, effectiveness and safety in the treatment of multiple sclerosis

Introducción. Existe evidencia creciente de que las células B y la inmunidad humoral tienen un papel fundamental en la fisiopatogenia de la esclerosis múltiple (EM). El ocrelizumab, un anticuerpo monoclonal anti-CD20, ha demostrado ser eficaz en el control de la enfermedad y recientemente ha sido aprobado por la Food and Drug Administration estadounidense para el tratamiento de las formas primariamente progresivas y las formas recidivantes de la EM. A la espera de su comercialización, el uso del rituximab, con un mecanismo de acción similar, se ha expandido ampliamente en el área de las enfermedades desmielinizantes. Objetivo. Abordar los principales aspectos de eficacia, efectividad y seguridad del rituximab en el tratamiento de la EM. Desarrollo. Se realizó una revisión bibliográfica a través de PubMed de los ensayos clínicos controlados con placebo, los estudios prospectivos abiertos, los estudios observacionales retrospectivos y las series de casos que utilizaron rituximab en poblaciones adultas afectas de EM. Se valoró su impacto en el control clínico y radiológico de la enfermedad, así como los aspectos relevantes de seguridad. Conclusiones. En todos los estudios revisados, el rituximab demostró un beneficio consistente en cuanto al control de la actividad inflamatoria, tanto clínica, reduciendo la incidencia de brotes, como radiológica, evitando la aparición de lesiones nuevas o activas. Por el contrario, respecto a la progresión de la discapacidad, su efecto es más controvertido. No se hallaron alertas de seguridad destacables. El rituximab parece ser un fármaco eficaz, efectivo y seguro en el tratamiento de la EM (AU)

Introduction. There is increasing evidence that B cells and humoral immunity play key roles in the pathogenesis of multiple sclerosis (MS). Ocrelizumab, an anti-CD20 monoclonal antibody, has been shown to be effective in controlling the disease and has recently been aproved by the Food and Drug Administration for the treatment of primary progressive and relapsing MS. While awaiting its marketing authorization, the use of rituximab, with a similar mechanism of action, has expanded widely in the area of demyelinating diseases. Aim. To address the main aspects of efficacy, effectiveness and safety of rituximab in the treatment of MS. Development. PubMed review of placebo-controlled clinical trials, prospective open label studies, retrospective observational studies, and case series using rituximab in adult MS affected populations were performed. Its impact on the clinical and radiological control of the disease was evaluated, as well as any relevant safety issues. Conclusions. In all of the studies reviewed, rituximab demonstrated a consistent benefit in controlling inflammatory activity, both clinically, reducing the incidence of relapses, and radiologically, avoiding the appearance of new and/or active lesions. On the contrary, with regards to the progression of disability, its effect is more controversial. Safety profile appears acceptable. Rituximab seems to be an effective and safe drug in the treatment of MS (AU)

A five year prospective investigation of anterior pituitary function after traumatic brain injury: is hypopituitarism long-term after head trauma associated with autoimmunity?

Traumatic brain injury (TBI) has been recently recognized as a common cause of pituitary dysfunction. However, there are not sufficient numbers of prospective studies to understand the natural history of TBI induced hypopituitarism. The aim was to report the results of five years' prospective follow-up of anterior pituitary function in patients with mild, moderate and severe TBI. Moreover, we have prospectively investigated the associations between TBI induced hypopituitarism and presence of anti-hypothalamus antibodies (AHA) and anti-pituitary antibodies (APA). Twenty five patients (20 men, five women) were included who were prospectively evaluated 12 months and five years after TBI, and 17 of them also had a third-year evaluation. Growth hormone (GH) deficiency is the most common pituitary hormone deficit at one, three, and five years after TBI. Although most of the pituitary hormone deficiencies improve over time, there were substantial percentages of pituitary hormone deficiencies at the fifth year (28% GH, 4% adrenocorticotropic hormone [ACTH], and 4% gonadotropin deficiencies). Pituitary dysfunction was significantly higher in strongly AHA- and APA-positive (titers ≥1/16) patients at the fifth year. In patients with mild and moderate TBI, ACTH and GH deficiencies may improve over time in a considerable number of patients but, although rarely, may also worsen over the five-year period. However in severe TBI, ACTH and GH status of the patients at the first year evaluation persisted at the fifth year. Therefore, screening pituitary function after TBI for five years is important, especially in patients with mild TBI. Moreover, close strong associations between the presence of high titers of APA and/or AHA and hypopituitarism at the fifth year were shown for the first time.

Predictive role of the immunostaining pattern of immunofluorescence and the titers of antipituitary antibodies at presentation for the occurrence of autoimmune hypopituitarism in patients with autoimmune polyendocrine syndromes over a five-year follow-up.

CONTEXT: Antipituitary antibodies (APA) are frequently present in patients with autoimmune polyendocrine syndrome (APS). DESIGN: The aim was to evaluate the predictive value of APA for the occurrence of hypopituitarism. A total of 149 APA-positive and 50 APA-negative patients with APS and normal pituitary function were longitudinally studied for 5 yr. METHODS: APA, by indirect immunofluorescence, and anterior pituitary function were assessed yearly in all patients. The risk for developing autoimmune pituitary dysfunction was calculated using survival and multivariate analysis. RESULTS: Hypopituitarism occurred in 28 of 149 (18.8%) APA-positive patients but in none of the 50 APA-negative patients. The immunostaining pattern in APA-positive patients involved either isolated pituitary cells [type 1 pattern; n=99 (66.4%)] or all pituitary cells [type 2 pattern; n=50 (33.6%)]. All patients developing pituitary dysfunction throughout the study span had a type 1 pattern. Kaplan-Meier curves for cumulative survival showed a significantly higher rate for developing hypopituitarism in relation to positive APA tests (P<0.005), pattern of immunostaining (P<0.0001), and APA titers (P<0.000001). Cox regression analysis in APA-positive patients with a type 1 pattern demonstrated a significantly (P<0.0001) higher risk for the onset of hypopituitarism in relation to increasing titers of APA. CONCLUSIONS: APA measurement by immunofluorescence may help to predict the occurrence of hypopituitarism but only when considering the immunostaining pattern and their titers. Combined evaluation of these parameters allows identifying patients at higher risk for pituitary autoimmune dysfunction, thus requiring a strict pituitary surveillance to disclose a preclinical phase of hypopituitarism and possibly interrupt therapeutically the progression to clinically overt disease.

Dual roles of the central domain of colicin D tRNase in TonB-mediated import and in immunity.

Colicin D import into Escherichia coli requires an interaction via its TonB box with the energy transducer TonB. Colicin D cytotoxicity is inhibited by specific tonB mutations, but it is restored by suppressor mutations in the TonB box. Here we report that there is a second site of interaction between TonB and colicin D, which is dependent upon a 45-amino acid region, within the uncharacterized central domain of colicin D. In addition, the 8th amino acids of colicin D (a glycine) and colicin B (a valine), adjacent to their TonB boxes, are also required for TonB recognition, suggesting that high affinity complex formation involves multiple interactions between these colicins and TonB. The central domain also contributes to the formation of the immunity complex, as well as being essential for uptake and thus killing. Colicin D is normally secreted in association with the immunity protein, and this complex involves the following two interactions: a major interaction with the C-terminal tRNase domain and a second interaction involving the central domain of colicin D and, most probably, the alpha4 helix of ImmD, which is on the opposite side of ImmD compared with the major interface. In contrast, formation of the immunity complex with the processed cytotoxic domain, the form expected to be found in the cytoplasm after colicin D uptake, requires only the major interaction. Klebicin D has, like colicin D, a ribonuclease activity toward tRNAArg and a central domain, which can form a complex with ImmD but which does not function in TonB-mediated transport.