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Chapter 47 : The Legionella pneumophila Dot/Icm Type IV Secretion System
Category: Bacterial Pathogenesis
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This chapter focuses on one major class of virulence factors, the dot/icm genes. The initial set of dot/icm genes was independently identified via a plate selection and an enrichment strategy in the Shuman and Isberg laboratories, respectively. Type IV secretion systems consist of both plasmid transfer systems and adapted conjugation systems used by pathogens to export substrates. Coxiella burnetii, the causative agent of Q fever, also contains a type IVB secretion system that strongly resembles the Legionella pneumophila Dot/Icm T4SS. It was initially believed, based on the homology to a plasmid transfer system, that the L. pneumophila T4SS might transfer a DNA substrate into the host cell similar to the plant pathogen Agrobacterium tumefaciens. The L. pneumophila Dot/Icm T4SS has the ability to transfer plasmids between bacterial cells and can export a variety of protein substrates into eukaryotic host cells.
The dot/icm genes (shown with black arrows) are located in two regions on the L. pneumophila chromosome. The white arrows indicate genes that are not required for intracellular growth. The appropriate dot or icm name is shown above the arrow except in the case where the gene has two names and then only the dot name is indicated.
Schematic representing predicted locations and possible interactions between the Dot/Icm proteins. The proteins are predicted to localize to the cytoplasm, the inner membrane, the periplasm, or the outer membrane. Known interactions include the following pairs: IcmQ–IcmR, IcmS–IcmW, and DotU–IcmF.
Characteristics of the L. pneumophila Dot/Icm proteins