Co-ordination of Legionella pneumophila virulence with entry into stationary phase by ppGpp

Legionella pneumophila survives in aquatic environments, but replicates wit hin amoebae or the alveolar macrophages of immunocompromised individuals. H ere, the signal transduction pathway that co-ordinates L. pneumophila virul ence expression in response to amino acid depletion was investigated. To fa cilitate kinetic and genetic studies, a phenotypic reporter of virulence wa s engineered by fusing flaA promoter sequences to a gene encoding green flu orescent protein. When subjected to amino acid depletion, L. pneumophila ac cumulated ppGpp and converted from a replicative to a virulent state, as ju dged by motility and sodium sensitivity, ppGpp appeared to initiate this re sponse, as L. pneumophila induced to express the Escherichia coli RelA ppGp p synthetase independently of nutrient depletion accumulated ppGpp, exited the exponential growth phase and expressed flaAgfp, motility, sodium sensit ivity, cytotoxicity and infectivity, five traits correlated with virulence. Although coincident with the stationary phase, L. pneumophila virulence ex pression appeared to require an additional factor: mutant Lp120 accumulated ppGpp and acquired two stationary phase traits but none of six virulence p henotypes analysed. We propose that, when nutrients are limiting, ppGpp act s as an alarmone, triggering the expression of multiple traits that enable L. pneumophila to escape its spent host, to survive and disperse in the env ironment and to re-establish a protected intracellular replication niche.