More than 16,000 effectors in theLegionellagenus genome provide multiple, independent combinations for replication in human cells
Authors: Gomez-Valero Laura , Rusniok Christophe , Carson Danielle , Mondino Sonia , Pérez-Cobas Ana Elena , Rolando Monica , Pasricha Shivani , Reuter Sandra , Demirtas Jasmin , Crumbach Johannes , Descorps-Declere Stephane , Hartland Elizabeth L. , Jarraud Sophie , Dougan Gordon , Schroeder Gunnar N. , Frankel Gad , Buchrieser Carmen 
Published: June 2018

Significance

Legionella pneumophilais a bacterial pathogen causing outbreaks of a lethal pneumonia. The genusLegionellacomprises 65 species for which aquatic amoebae are the natural reservoirs. Using functional and comparative genomics to deconstruct the entire bacterial genus we reveal the surprising parallel evolutionary trajectories that have led to the emergence of human pathogenicLegionella.An unexpectedly large and unique repository of secreted proteins (>16,000) containing eukaryotic-like proteins acquired from all domains of life (plant, animal, fungal, archaea) is contrasting with a highly conserved type 4 secretion system. This study reveals an unprecedented environmental reservoir of bacterial virulence factors, and provides a new understanding of how reshuffling and gene-acquisition from environmental eukaryotic hosts, may allow for the emergence of human pathogens.

The bacterial genusLegionellacomprises 65 species among, whichLegionella pneumophilais a human pathogen causing severe pneumonia. To understand the evolution of an environmental to an accidental human pathogen, we have functionally analyzed 80Legionellagenomes spanning 58 species. Uniquely, an immense repository of 16,000 secreted proteins encoding 137 different eukaryotic-like domains and more than 200 eukaryotic-like proteins is paired with a highly conserved T4SS. Specifically, we show that eukaryotic Rho and Rab GTPase domains are found nearly exclusively in eukaryotes andLegionellaspecies. Translocation assays for selected Rab-GTPase proteins revealed that they are indeed T4SS secreted substrates. Furthermore, F/U-box and SET domains were present in >70% of all species suggesting that manipulation of host signal transduction, protein turnover and chromatin modification pathways, respectively are fundamental intracellular replication strategies forLegionellae. In contrast, the Sec-7 domain was restricted toL. pneumophilaand seven other species, indicating effector repertoire tailoring within different amoebae. Functional screening of 47 species revealed 60% were competent for intracellular replication in THP-1 cells, but interestingly this phenotype was associated with diverse effector assemblages. These data, combined with evolutionary analysis indicate that the capacity to infect eukaryotic cells has been acquired independently many times within the genus and that a highly conserved yet versatile T4SS secretes an exceptional number of different proteins shaped by inter-domain gene transfer. Furthermore we revealed the surprising extent to which legionellae have co-opted genes and thus cellular functions from their eukaryotic hosts and provides a new understanding of how dynamic reshuffling and gene-acquisition has led to the emergence of major human pathogens.