Cloning and characterisation of the Pasteurella multocida ahpA gene responsible for a haemolytic phenotype in Escherichia coli

Cache Domains That are Homologous to, but Different from PAS Domains Comprise the Largest Superfamily of Extracellular Sensors in Prokaryotes

Cellular receptors usually contain a designated sensory domain that recognizes the signal. Per/Arnt/Sim (PAS) domains are ubiquitous sensors in thousands of species ranging from bacteria to humans. Although PAS domains were described as intracellular sensors, recent structural studies revealed PAS-like domains in extracytoplasmic regions in several transmembrane receptors. However, these structurally defined extracellular PAS-like domains do not match sequence-derived PAS domain models, and thus their distribution across the genomic landscape remains largely unknown. Here we show that structurally defined extracellular PAS-like domains belong to the Cache superfamily, which is homologous to, but distinct from the PAS superfamily. Our newly built computational models enabled identification of Cache domains in tens of thousands of signal transduction proteins including those from important pathogens and model organisms. Furthermore, we show that Cache domains comprise the dominant mode of extracellular sensing in prokaryotes.

Cell-surface receptors control multiple cellular functions and are attractive targets for drug design. These receptors often have dedicated extracellular domains that bind signaling molecules, such as hormones and nutrients. Computational identification of these ligand-binding domains in genomic sequences is a pre-requisite for their further experimental characterization. Using available three-dimensional structures of several bacterial cell-surface receptors, we built computational models that enabled identification of the Cache domain, as the most common extracellular sensor module in prokaryotes, including many important pathogens. We also demonstrated that the Cache domain is homologous to, but sufficiently different from the most common intracellular sensor module, the PAS domain. These findings provide a unified view on molecular principles of signal recognition by extra- and intracellular receptors.

Profiling of virulence associated genes of Pasteurella multocida isolated from cattle

Pasteurella multocida is a causative agent of many major diseases of which haemorrhagic septiciemia (HS) in cattle & a buffalo is responsible for significant losses to livestock sector in India and south Asia. The disease outcome is affected by various host- and pathogen-specific determinants. Several bacterial species-specific putative virulence factors including the capsular and virulence associated genes have been proposed to play a key role in this interaction. A total of 23 isolates of P. multocida were obtained from 335 cases of various clinically healthy and diseased cattle. These isolates were examined for capsule synthesis genes (capA, B, D, E and F) and eleven virulence associated genes (tbpA, pfhA, toxA, hgbB, hgbA, nanH, nanB, sodA, sodC, oma87 and ptfA) by PCR. A total of 19 P. multocida isolates belonging to capsular type B and 4 of capsular type A were isolated. All isolates of capsular type B harboured the virulence associated genes: tbpA, pfhA, hgbA, sodC and nanH, coding for transferrin binding protein, filamentous hemagglutinin, haemoglobin binding protein, superoxide dismutase and neuraminidases, respectively; while isolates belonging to capsular type A also carried tbpA, pfhA, hgbA and nanH genes. Only 50 % of capsular type A isolates contained sodC gene while 100 % of capsular type B isolates had sodC gene. The gene nanB and toxA were absent in all the 23 isolates. In capsular type A isolates, either sodA or sodC gene was present & these genes did not occur concurrently. The presence of virulence associated gene ptfA revealed a positive association with the disease outcome in cattle and could therefore be an important epidemiological marker gene for characterizing P. multocida isolates.

Cloning and characterisation of the ahpA gene of Pasteurella multocida serogroup b:2 (strain P52): short communication

Pasteurella multocida B:2 is responsible for haemorrhagic septicaemia in cattle and buffaloes, causing severe economic losses in the developing countries. In the present study, the ahpA gene of P. multocida B:2 (P52) was cloned, sequenced and compared with the previously reported ahpA gene sequence in P. multocida A:1, which is responsible for its haemolytic phenotype. E. coli DH5a cells were further transformed with recombinant plasmid carrying the ahpA gene from P. multocida B:2 (P52) but SDS-PAGE analysis failed to show the expression of haemolysin protein. Slight haemolysis was albeit observed in horse blood agar plates streaked with recombinant E. coli carrying the ahpA gene. Our study indicates that there is 99.6% similarity and 0.4% divergence between ahpA gene of P. multocida B:2 (P52) and P. multocida A: 1, while membrane topology analysis has predicted that ahpA is an inner membrane protein with two strong hydrophobic regions at the N and C terminals. The presence of significant homology in ahpA sequence in A: 1 and B:2 perhaps suggests a common mechanism of pathogenesis in different species of animals.

Pasteurella multocida contains multiple immunogenic haemin- and haemoglobin-binding proteins

Iron-dependent outer membrane proteins (IROMPs) play an important role in bacterial pathogenesis and present several attributes of potential vaccine candidates. TBLASTN analysis of the Pasteurella multocida Pm70 genome using the same molecules of other bacterial pathogens as a query identified eight putative haemin and haemoglobin receptors for this organism. Quantitative binding assays have demonstrated that the proteins PM0040, PM0236, PM0741, PM1081, PM1428, PM0592 and HgbA bind both haemin and haemoglobin, whereas PM0576 and PM1282 ORFs only bind either haemoglobin or haemin, respectively. Furthermore, Western blot analysis showed that P. multocida-infected mice generate specific antibodies against PM0040, PM0236, PM0741, PM1081, PM1428, PM0592 and HgbA proteins. Nevertheless, inoculation of mice with any single one of these receptors alone did not protect against P. multocida infection.

Functional characterization of HgbB, a new hemoglobin binding protein of Pasteurella multocida

The biological function and role in pathogenesis of a Pasteurella multocida A:1 strain hemoglobin binding protein was investigated. The hgbB gene from the P. multocida A:1 strain, VP161, was cloned and characterized. hgbB was 2991 bp in length and encoded a mature length protein of 111 kDa. HgbB was predicted to be an outer membrane protein and shared 68 and 69% similarity to the hemoglobin/hemoglobin-haptoglobin binding protein, HI0712 from Haemophilus influenzae Rd and HgpC, from H. influenzae b, respectively. HgbB exhibited features typical of TonB dependent receptors, including seven conserved regions typical of these proteins, and conserved invariant residues. Escherichia coli expressing recombinant HgbB was found to bind hemoglobin in a solid phase dot blot binding assay. However, when a truncated form of the protein was expressed in E. coli, cells could no longer bind hemoglobin. Insertional inactivation of hgbB did not affect the ability of P. multocida to bind hemoglobin, nor its ability to produce disease in a mouse model. In addition, recombinant HgbB did not confer any protection against homologous or heterologous challenge.

Characterisation of a Pasteurella multocida esterase gene which confers a hemolytic phenotype in Escherichia coli under anaerobic conditions

Investigation of the hemolytic phenotype under anaerobic growth conditions of an avian Pasteurella multocida strain, PBA100, resulted in the identification and characterisation of a gene encoding an esterase enzyme, mesA, that conferred a hemolytic phenotype in Escherichia coli under anaerobic conditions. MesA appeared to be expressed and functional under anaerobic and aerobic conditions in both E. coli and P. multocida. A P. multocida mesA mutant was generated which resulted in the loss of acetyl esterase activity under anaerobic conditions. However, this mutation did not cause any attenuation of virulence for mice nor a detectable change to the anaerobic hemolytic phenotype of P. multocida. In E. coli MesA appeared to cause hemolysis indirectly by the induction of the latent E. coli K-12 cytolysin, sheA.