The DotL Protein, a Member of the TraG-Coupling Protein Family, Is Essential for Viability ofLegionella pneumophilaStrain Lp02

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Abstract
Legionella pneumophilais able to survive inside phagocytic cells by an internalization route that bypasses fusion of the nascent phagosome with the endocytic pathway to allow formation of a replicative phagosome. Thedot/icmgenes, a major virulence system ofL. pneumophila, encode a type IVB secretion system that is required for intracellular growth. One Dot protein, DotL, has sequence similarity to type IV secretion system coupling proteins (T4CPs). In other systems, coupling proteins are not required for viability of the organism. Here we report the first example of a strain,L. pneumophilaLp02, in which a putative T4CP is essential for viability of the organism on bacteriological media. This result is particularly surprising since the majority of thedot/icmgenes in Lp02 are dispensable for growth outside of a host cell, a condition that does not require a functional Dot/Icm secretion complex. We were able to isolate suppressors of theΔdotLlethality and found that many contained mutations in other components of the Dot/Icm secretion system. A systematic analysis ofdot/icmdeletion mutants revealed that the majority of them (20 of 26) suppressed the lethality phenotype, indicating a partially assembled secretion system may be the source ofΔdotLtoxicity in the wild-type strain. These results are consistent with a model in which the DotL protein plays a role in regulating the activity of theL. pneumophilatype IV secretion apparatus.