Affiliation:
1. Host-Parasite Interactions Section, Laboratory of Intracellular Parasites, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, Montana 59840
Abstract
ABSTRACT
Spotted fever group rickettsiae are known to produce distinct plaque phenotypes. Strains that cause lytic infections in cell culture form clear plaques, while nonlytic strains form opaque plaques in which the cells remain intact. Clear plaques have historically been associated with more-virulent species or strains of spotted fever group rickettsiae. We have selected spontaneous mutant pairs from two independent strains of
Rickettsia rickettsii
, the virulent R strain and the avirulent Iowa strain. A nonlytic variant of
R. rickettsii
R, which typically produces clear plaques, was isolated and stably maintained. A lytic variant of the Iowa strain, which characteristically produces opaque plaques, was also selected and maintained. Genomic resequencing of the variants identified only a single gene disrupted in each strain. In both cases, the mutation was in a gene annotated as
relA
/
spoT
-like. In the Iowa strain, a single mutation introduced a premature stop codon upstream from region encoding the predicted active site of RelA/SpoT and caused the transition to a lytic plaque phenotype. In
R. rickettsii
R, the nonlytic plaque phenotype resulted from a single-nucleotide substitution that shifted a tyrosine residue to histidine near the active site of the enzyme. The intact
relA
/
spoT
gene thus occurred in variants with the nonlytic plaque phenotype. Complementation of the truncated
relA
/
spoT
gene in the Iowa lytic plaque variant restored the nonlytic phenotype. The
relA
/
spoT
mutations did not affect the virulence of either strain in a Guinea pig model of infection; R strain lytic and nonlytic variants both induced fever equally, and the mutation in Iowa to a lytic phenotype did not cause them to become virulent.
Publisher
American Society for Microbiology
Subject
Infectious Diseases,Immunology,Microbiology,Parasitology
Cited by
28 articles.
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