The Pasteurella multocida txaR gene is expressed in Escherichia coli but does not repress transcription of the P. multocida toxin gene toxA

Hydrophobins are required for conidial hydrophobicity and plant root colonization in the fungal biocontrol agent Clonostachys rosea

BACKGROUND

Filamentous fungi produce small cysteine rich surface active amphiphilic hydrophobins on the outer surface of cell walls that mediate interactions between the fungus and the environment. The role of hydrophobins in surface hydrophobicity, sporulation, fruit body formation, recognition and adhesion to host surface and virulence have been reported. The aim of the present study was to characterize the biological function of hydrophobins in the fungal biocontrol agent Clonostachys rosea in order to understand their potential roles in biocontrol mechanisms.

RESULTS

Based on the presence of hydrophobin domains, cysteine spacing patterns and hydropathy plots, we identified three class II hydrophobin genes in C. rosea. Gene expression analysis showed basal expression of Hyd1, Hyd2 and Hyd3 in all conditions tested with the exception of induced Hyd1 expression in conidiating mycelium. Interestingly, up-regulation of Hyd1, Hyd2 and Hyd3 was found during C. rosea self interaction compared to interactions with the fungal plant pathogens Botrytis cinerea or Fusarium graminearum in dual culture assays. Phenotypic analysis of C. rosea deletion and complementation strains showed that Hyd1 and Hyd3 are jointly required for conidial hydrophobicity, although no difference in mycelia hydrophobicity was found between wild type (WT) and mutant strains. Interestingly, mutant strains showed increased growth rates, conidiation and enhanced tolerances of conidia to abiotic stresses. Antagonism tests using in vitro dual culture and detached leaf assays showed that the mutant strains were more aggressive towards B. cinerea, F. graminearum or Rhizoctonia solani, and that aggression was partly related to earlier conidial germination and enhanced tolerance of mutant strains to secreted fungal metabolites. Furthermore, in vitro Arabidopsis thaliana root colonization assays revealed reduced root colonization ability of the ΔHyd3 strain, but not for the ΔHyd1 strain. Furthermore, enhanced root colonization ability for the ΔHyd1ΔHyd3 strain was found in comparison to WT.

CONCLUSIONS

These results show a role for hydrophobins in conidial hydrophobicity, control of conidial germination under stress conditions, and in root colonization in C. rosea. However, functional studies of Hyd2 remains to be performed in order to fully assess the role of hydrophobins in C. rosea.

Autodisplay: one-component system for efficient surface display and release of soluble recombinant proteins from Escherichia coli

The immunoglobulin A protease family of secreted proteins are derived from self-translocating polyprotein precursors which contain C-terminal domains promoting the translocation of the N-terminally attached passenger domains across gram-negative bacterial outer membranes. Computer predictions identified the C-terminal domain of the Escherichia coli adhesin involved in diffuse adherence (AIDA-I) as a member of the autotransporter family. A model of the beta-barrel structure, proposed to be responsible for outer membrane translocation, served as a basis for the construction of fusion proteins containing heterologous passengers. Autotransporter-mediated surface display (autodisplay) was investigated for the cholera toxin B subunit and the peptide antigen tag PEYFK. Up to 5% of total cellular protein was detectable in the outer membrane as passenger autotransporter fusion protein synthesized under control of the constitutive P(TK) promoter. Efficient presentation of the passenger domains was demonstrated in the outer membrane protease T-deficient (ompT) strain E. coli UT5600 and the ompT dsbA double mutant JK321. Surface exposure was ascertained by enzyme-linked immunosorbent assay, immunofluorescence microscopy, and immunogold electron microscopy using antisera specific for the passenger domains. In strain UT2300 (ompT+), the passenger domains were released from the cell surface by the OmpT protease at a novel specific cleavage site, R / V. Autodisplay represents a useful tool for future protein translocation studies with interesting biotechnological possibilities.