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J. Bacteriol., 03 1997, 1533-1540, Vol 179, No. 5
MT Ripio, G Dominguez-Bernal, M Lara, M Suarez and JA Vazquez-Boland
Virulence genes in Listeria monocytogenes are coordinately expressed under
the control of the transcriptional activator PrfA, encoded by prfA, a
member of the cyclic AMP (cAMP) receptor protein (CRP)/FNR family of
bacterial regulators. Strain P14-A is a spontaneous mutant of L.
monocytogenes serovar 4b which produces elevated levels of virulence
factors (M. T. Ripio, G. Dominguez-Bernal, M. Suarez, K. Brehm, P. Berche,
and J. A. Vazquez-Boland, Res. Microbiol. 147:371-384, 1996). Here we
report that P14-A and other variant strains with the same phenotype carry a
point mutation in codon 145 of prfA, leading to a Gly- ->Ser
substitution. trans-complementation experiments with PrfA- deficient
mutants demonstrated that the Gly145Ser prfA allele causes overexpression
of virulence factors in L. monocytogenes, to the levels found in the
virulence factor-overexpressing variants. In strain P14-A with a
chromosomal Glyl45Ser prfA background, transcription of prfA and of
PrfA-dependent virulence genes remained constitutively high under culture
conditions in which virulence factor expression is downregulated in
wild-type L. monocytogenes. The Glyl45Ser substitution is located in a PrfA
stretch (residues 141 to 151) showing high sequence similarity to the D
alpha-helix of CRP. Interestingly, well- characterized crp* mutations,
which make CRP functionally active in the absence of cAMP, map in this
region (i.e., Gly141Ser and Ala144Thr substitutions). By analogy with the
CRP model, the phenotype conferred to L. monocytogenes by the Gly145Ser
substitution in PrfA could be due to the mutant regulatory protein being
locked in a transcriptionally active, cofactor-independent conformational
state. Our observations allow the construction of a model for
PrfA-dependent virulence gene regulation in which the levels of virulence
factor expression depend primarily on the conformational state of the PrfA
protein, which alternates between active and inactive forms according to
its interaction with an environmentally regulated signal molecule or
cofactor.
Copyright © 1997, American Society for Microbiology
A Gly145Ser substitution in the transcriptional activator PrfA causes constitutive overexpression of virulence factors in Listeria monocytogenes
Unidad de Microbiologia e Immunologia, Facultad de Veterinaria, Universidad Complutense, Madrid, Spain.
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