A Nutrient-Responsive LuxR Regulator Orchestrates Effector Gene Expression Across the Legionella Genus.

Legionella pneumophila utilizes the Icm/Dot secretion system to translocate > 330 effectors into host cells, yet the regulatory mechanisms controlling the expression of many effector-encoding genes (EEGs) remain unknown. Here, we identify and characterize LexR1 (lpg2524), a LuxR-type transcriptional regulator that controls EEG expression in L. pneumophila and other Legionella species. In L. pneumophila, LexR1 directly activates the expression of four EEGs (lem26, lem16, ravW, and legC1) by binding a conserved inverted repeat located upstream of these genes. We demonstrate that LexR1 activity is stimulated by a nutrient-derived ligand present in casamino acids, and orthologs from other Legionella species exhibit similar casamino acid dependent activation of their target genes. Genomic analyses identified 29 genes containing the LexR1 regulatory element in the seven Legionella species containing LexR1, most of which encode validated effectors or proteins harboring common effector domains. Many of these genes are shared among the species and appear to have co-transferred with their regulatory regions via horizontal gene transfer. LexR1 expression is positively regulated by RpoS and repressed by Fis, linking it to the broader effector regulatory network. Together, these findings identify LexR1 as a regulator of EEGs that coordinates nutrient sensing with effector gene expression across the Legionella genus.