Late embryogenesis abundant (LEA) proteins are important players in the management of responses to stressful conditions, such as drought, high salinity, and changes in temperature. Many LEA proteins do not have defined three-dimensional structures, so they are intrinsically disordered proteins (IDPs) and are often highly hydrophilic. Although LEA-like sequences have been identified in bacterial genomes, the functions of bacterial LEA proteins have been studied only recently. Sequence analysis of outer membrane interleukin receptor I (BilRI) from the oral pathogen
Aggregatibacter actinomycetemcomitans
indicated that it shared sequence similarity with group 3/3b/4 LEA proteins. Comprehensive nuclearcgq
magnetic resonance (NMR) studies confirmed its IDP nature, and expression studies in
A. actinomycetemcomitans
harboring a red fluorescence reporter protein-encoding gene revealed that
bilRI
promoter expression was increased at decreased temperatures. The amino acid backbone of BilRI did not stimulate either the production of reactive oxygen species from human leukocytes or the production of interleukin-6 from human macrophages. Moreover, BilRI-specific IgG antibodies could not be detected in the sera of
A. actinomycetemcomitans
culture-positive periodontitis patients. Since the
bilRI
gene is located near genes involved in natural competence (i.e., genes associated with the uptake of extracellular (eDNA) and its incorporation into the genome), we also investigated the role of BilRI in these events. Compared to wild-type cells, the Δ
bilRI
mutants showed a lower transformation efficiency, which indicates either a direct or indirect role in natural competence. In conclusion,
A. actinomycetemcomitans
might express BilRI, especially outside the host, to survive under stressful conditions and improve its transmission potential.
Biofilm formation contributes to the virulence of various pathogens, as the extracellular polymer matrix provides protection against the host immune defense and antimicrobial drugs. Biofilm-associated diseases often become chronic and recurring. The periodontal pathogen Aggregatibacter actinomycetemcomitans, which resides in a multispecies biofilm in the subgingival pocket, produces multiple virulence factors that can contribute to disease progression. Certain strains of the species are naturally competent, which allows uptake of extracellular DNA that can be incorporated into the bacterial genome or used as a nutrient. Earlier studies indicated that bacterial interleukin receptor I (BilRI) and the type IV pilus subunit PilA protein are needed for efficient transformation in A. actinomycetemcomitans. In this study, we show that the outer membrane secretin HofQ is required for natural competence, as deletion of the hofQ gene results in a nontransformable strain. Furthermore, we studied the gene expression profiles of three single-gene mutants of the naturally competent A. actinomycetemcomitans strain D7S, all of which have decreased transformation efficiency compared to the wild-type strain. Additionally, as A. actinomycetemcomitans can bind to and internalize interleukin (IL)-1β, the effect of IL-1β on bacterial gene expression was also studied. However, in our experimental setup, the addition of IL-1β did not change gene expression in the A. actinomycetemcomitans strains used. The mutant strain lacking the bilRI gene exhibited a gene expression pattern similar to that of the wild-type strain. However, deletion of hofQ or pilA resulted in altered gene expression. Interestingly, genes associated with anaerobic growth, biofilm formation, and virulence were downregulated in the ΔhofQ and ΔpilA deletion mutants, which could indicate a decreased colonization ability and reduced virulence.
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