Quorum sensing

Identification of the central quorum sensing regulator of virulence in the enteric phytopathogen Erwinia carotovora : the VirR repressor Burr, T. et al. Mol. Microbiol. 59, 113–430 (2006)

In Erwinia carotovora, quorum sensing (QS) is known to have a central role in regulating virulence-gene expression. Homologues of the components of the prototypic Vibrio fischeri QS system, the acyl-homoserine lactone (AHL) synthase LuxI and the response regulator LuxR, have been identified in E. carotovora but, until now, the identity of the regulator that prevents the QS-controlled virulence factors from being produced at low cell density has been unknown. Tom Burr and colleagues have provided this missing link by identifying a novel virulence regulator, VirR, in E. carotovora subsp. atroseptica and demonstrating that VirR negatively regulates the production of exoenzymes in the absence of the Erwinia QS signalling molecule.

Techniques & applications

An ordered, nonredundant library of Pseudomonas aeruginosa strain PA14 transposon insertion mutants Liberati, N. T. et al. Proc. Natl Acad. Sci. USA 103, 2833–2838 (2006)

Frederick Ausubel and colleagues report in PNAS on the construction of a non-redundant transposon-insertion library for Pseudomonas aeruginosa strain PA14 using a derivative of Himar1, a member of the mariner transposon family. The library allows the P. aeruginosa genome to be scanned using just 5,459 mutants, and so will be invaluable for high-throughput screening of the P. aeruginosa genome. An accompanying database (the PA14 transposon insertion mutant database, PATIMD) has been developed to facilitate mutant sorting and analysis, and is publicly available at http://ausubellab.mgh.harvard.edu/cgi-bin/pa14/home.cgi.

Malaria

A var gene promoter controls allelic exclusion of virulence genes in Plasmodium falciparum malaria Voss, T. S. et al. Nature 439, 1004–1008 (2006)

A model outlining how the malaria parasite Plasmodium falciparum controls antigenic variation of the key virulence factor PfEMP1 has been proposed in a recent issue of Nature. PfEMP1 is encoded by the var gene family, which contains 60 var genes, each of which encodes a distinct PfEMP1 protein. Only one var gene is transcribed in each P. falciparum parasite at any one time, facilitating antigenic variation of PfEMP1, but the allelic exclusion mechanism involved has been unclear until now, although it was known that epigenetic regulation was involved. Voss et al. propose that allelic exclusion is controlled by a var promoter sequence, activation of which is sufficient to initiate transcription of one var gene while ensuring that the other 59 var genes are silenced, and suggest that activation occurs only in a particular 'expression spot' in a perinuclear compartment.