Chromosomal insertion and excision of a 30 kb unstable genetic element is responsible for phase variation of lipopolysaccharide and other virulence determinants in Legionella pneumophila

Citation
E. Luneberg et al., Chromosomal insertion and excision of a 30 kb unstable genetic element is responsible for phase variation of lipopolysaccharide and other virulence determinants in Legionella pneumophila, MOL MICROB, 39(5), 2001, pp. 1259-1271
Citations number
57
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Microbiology
Journal title
MOLECULAR MICROBIOLOGY
ISSN journal
0950-382X → ACNP
Volume
39
Issue
5
Year of publication
2001
Pages
1259 - 1271
Database
ISI
SICI code
0950-382X(200103)39:5<1259:CIAEOA>2.0.ZU;2-P
Abstract
We recently described the phase-variable expression of a virulence-associat ed lipopolysaccharide (LPS) epitope in Legionella pneumophila. In this stud y, the molecular mechanism for phase variation was investigated. We identif ied a 30 kb unstable genetic element as the molecular origin for LPS phase variation. Thirty putative genes were encoded on the 30 kb sequence, organi zed in two putative opposite transcription units. Some of the open reading frames (ORFs) shared homologies with bacteriophage genes, suggesting that t he 30 kb element was of phage origin. In the virulent wild-type strain, the 30 kb element was located on the chromosome, whereas excision from the chr omosome and replication as a high-copy plasmid resulted in the mutant pheno type, which is characterized by alteration of an LPS epitope and loss of vi rulence. Mapping and sequencing of the insertion site in the genome reveale d that the chromosomal attachment site was located in an intergenic region flanked by genes of unknown function. As phage release could not be induced by mitomycin C, it is conceivable that the 30 kb element is a non-function al phage remnant. The protein encoded by ORF T on the 30 kb plasmid could b e isolated by an outer membrane preparation, indicating that the genes enco ded on the 30 kb element are expressed in the mutant phenotype. Therefore, it is conceivable that the phenotypic alterations seen in the mutant depend on high-copy replication of the 30 kb element and expression of the encode d genes. Excision of the 30 kb element from the chromosome was found to occ ur in a RecA-independent pathway, presumably by the involvement of RecE, Re cT and RusA homologues that are encoded on the 30 kb element.