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Applied and Environmental Microbiology, December 2007, p. 7622-7628, Vol. 73, No. 23
0099-2240/07/$08.00+0     doi:10.1128/AEM.01000-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

A Metalloprotease Secreted by the Insect Pathogen Photorhabdus luminescens Induces Melanization{triangledown}

Kiara G. Held,1 Christopher N. LaRock,1 David A. D'Argenio,1 Celeste A. Berg,2 and Carleen M. Collins1*

Department of Microbiology,1 Genome Sciences, University of Washington, Seattle, Washington2

Received 3 May 2007/ Accepted 30 September 2007

Photorhabdus luminescens is a gram-negative insect pathogen that enters the hemocoel of infected hosts and produces a number of secreted proteins that promote colonization and subsequent death of the insect. In initial studies to determine the exact role of individual secreted proteins in insect pathogenesis, concentrated culture supernatants from various P. luminescens strains were injected into the tobacco hornworm Manduca sexta. Culture supernatants from P. luminescens TT01, the genome-sequenced strain, stimulated a rapid melanization reaction in M. sexta. Comparison of the profiles of secreted proteins from the various Photorhabdus strains revealed a single protein of approximately 37 kDa that was significantly overrepresented in the TT01 culture supernatant. This protein was purified by DEAE ion-exchange and Superdex 75 gel filtration chromatography and identified by matrix-assisted laser desorption ionization-time of flight analysis as the product of the TT01 gene plu1382 (NCBI accession number NC_005126); we refer to it here as PrtS. PrtS is a member of the M4 metalloprotease family. Injection of PrtS into larvae of M. sexta and Galleria mellonella and into adult Drosophila melanogaster and D. melanogaster melanization mutants (Bc) confirmed that the purified protein induced the melanization reaction. The prtS gene was transcribed by P. luminescens injected into M. sexta before death of the insect, suggesting that the protein was produced during infection. The exact function of this protease during infection is not clear. The bacteria might survive inside the insect despite the melanization process, or it might be that the bacterium is specifically activating melanization in an attempt to circumvent this innate immune response.

* Corresponding author. Mailing address: Department of Microbiology, University of Washington, Box 357242, Seattle, WA 98195. Phone: (206) 616-0581. Fax: (206) 543-8297. E-mail: carleen{at}u.washington.edu

{triangledown} Published ahead of print on 12 October 2007.

Applied and Environmental Microbiology, December 2007, p. 7622-7628, Vol. 73, No. 23
0099-2240/07/$08.00+0     doi:10.1128/AEM.01000-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.





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