A horizontally acquired Legionella genomic island encoding a LuxR type regulator and effector proteins displays variation in gene content and regulation

• Published in: Molecular microbiology Vol. 116; no. 3; pp. 766 - 782
• Main Authors: Linsky, Marika, Segal, Gil
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.09.2021
• Subjects: Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Effector cells; Effectors; Electrophoretic mobility; Factor For Inversion Stimulation Protein - genetics; Factor For Inversion Stimulation Protein - metabolism; Gene expression; Gene Expression Regulation, Bacterial; Gene transfer; Gene Transfer, Horizontal; Genes; Genome, Bacterial; genomic island; Genomic Islands; Genomics; horizontal gene transfer; Horizontal transfer; Humans; Legionella; Legionella pneumophila - genetics; Legionella pneumophila - metabolism; Legionnaires' Disease - microbiology; Legionnaires' disease bacterium; Mutagenesis; Proteins; Regulatory Sequences, Nucleic Acid; Repressor Proteins - genetics; Repressor Proteins - metabolism; Repressors; Trans-Activators - genetics; Trans-Activators - metabolism; Transcription; Yeast; Yeasts; effectors; Legionella; horizontal gene transfer; gene expression; genomic island
• ISSN: 0950-382X; 1365-2958
• DOI: 10.1111/mmi.14770

The intracellular pathogen Legionella pneumophila translocates >300 effector proteins into host cells, many of which are regulated at the transcriptional level. Here, we describe a novel L. pneumophila genomic island, which undergoes horizontal gene transfer within the Legionella genus. This island encodes two Icm/Dot effectors: LegK3 and a previously uncharacterized effector which we named CegK3, as well as a LuxR type regulator, which we named RegK3. Analysis of this island in different Legionella species revealed a conserved regulatory element located upstream to the effector‐encoding genes in the island. Further analyses, including gene expression analysis, mutagenesis of the RegK3 regulatory element, controlled expression studies, and gel‐mobility shift assays, all demonstrate that RegK3 directly activates the expression levels of legK3 and cegK3 effector‐encoding genes. Additionally, the expression of all the components of the island is silenced by the Fis repressors. Comparison of expression profiles of these three genes among different Legionella species revealed variability in the activation levels mediated by RegK3, which were positively correlated with the Fis‐mediated repression. Furthermore, LegK3 and CegK3 effectors moderately inhibit yeast growth, and importantly, they have a strong synergistic inhibitory effect on yeast growth, suggesting these two effectors are not only co‐regulated but also might function together. We describe a novel horizontally acquired genomic island which encodes a LuxR type regulator (RegK3) and two effector‐encoding genes (LegK3 and CegK3) found in several members of the Legionella genus. Our data provides insights into the expression, evolution and function of this genomic island. Our results suggest a strategy by which members of the Legionella genus develop and expand their effector repertoire and their effector regulatory network.