Actinomyces naeslundii is a pioneer of the oral cavity and forms a biofilm on the tooth's surface. Most bacteria require
iron for survival and in pathogenic bacteria
iron availability regulates virulence gene expression.
Metal-dependent repressors control gene expression involved in
metal transport and uptake including
siderophores.
Siderophores are small molecules synthesized by bacteria and fungi to acquire
iron. The A. naeslundii genome was searched for a gene encoding a
metal-dependent repressor. Actinomyces
metal-dependent repressor or amdR was identified. The AmdR
protein was examined for its ability to bind to the promoter sequence of a gene encoding the
siderophore uptake (
sid gene). According to gel shift assays, AmdR binds to the
sid gene promoter sequences. In the authors' model, when
iron is available AmdR binds to the
sid promoter and represses
sid gene expression. To further explore the role of AmdR, an amdR-defective strain of A. naeslundii was constructed and biofilm formation and
siderophore production were evaluated. When
iron is removed from the medium A. naeslundii increases biofilm and
siderophore production. However, amdR-defective A. naeslundii is less sensitive to
metal ion concentrations in the growth medium.