During its life cycle, Legionella pneumophila alternates between a replicative and a transmissive state. To determine their contributions to L. pneumophila differentiation, the two
ppGpp synthetases, RelA and SpoT, were disrupted. Synthesis of
ppGpp was required for transmission, as relA spoT mutants were killed during entry to and exit from macrophages. RelA, which senses
amino acid starvation induced by
serine hydroxamate, is dispensable in macrophages, as relA mutants spread efficiently. SpoT monitors
fatty acid biosynthesis (FAB), since following
cerulenin treatment, wild-type and relA strains expressed the flaA transmissive gene, but relA spoT mutants did not. As in Escherichia coli, the SpoT response to FAB perturbation likely required an interaction with
acyl-carrier protein (ACP), as judged by the failure of the spoT-A413E allele to rescue transmissive trait expression of relA spoT bacteria. Furthermore, SpoT was essential for transmission between macrophages, since
secondary infections by relA spoT mutants were restored by induction of spoT, but not relA. To resume replication,
ppGpp must be degraded, as mutants lacking spoT
hydrolase activity failed to convert from the transmissive to the replicative phase in either bacteriological medium or macrophages. Thus, L. pneumophila requires SpoT to monitor FAB and to alternate between replication and transmission in macrophages.