The Divergently Transcribed Streptococcus parasanguis Virulence-Associated fimA Operon Encoding an Mn ²⁺ -Responsive Metal Transporter and pepO Encoding a Zinc Metallopeptidase Are Not Coordinately Regulated

Abstract

The study of how bacteria respond to and obtain divalent metal ions provides insight into the regulation of virulence factors in the host environment. Regulation of metal permease operons in gram-positive bacteria may involve the binding of metal-responsive repressors to palindromic domains in their control regions. The Streptococcus parasanguis fimA operon, which encodes an ATP-binding cassette (ABC) transporter system with sequence homology to the LraI family of metal transporters, possesses a palindromic regulatory region with high homology to that of the Streptococcus gordonii ScaR binding domain. Mapping of the promoter and regulatory regions of fimA and the divergently transcribed pepO gene, which encodes a zinc metalloendopeptidase, indicated that their promoter and regulatory elements overlap. fimA had one transcriptional start site, whereas pepO had three. Analysis of truncated versions of the pepO promoter suggested that all three transcriptional start sites are functional. Analysis of promoter activity under various environmental conditions indicated that the fimA operon promoter and the pepO promoter are not coordinately regulated. Streptococcus parasanguis, along with other members of the mitis group of oral streptococci, are among some of the most successful colonizers of the human body. These commensal organisms in the oral cavity have the ability to attach, colonize, and thrive in an environment of continual flux of pH, temperature, mechanical stress, and nutrient availability. Introduction of these oral organisms into the bloodstream of individuals with predisposing heart valve damage can result in endocarditis, a life-threatening illness (3). Nutrients, in particular divalent metal ions, are often sequestered by the host in such a way that the colonizing bacteria must actively gain access to these resources in order to survive in the host environment. Iron in the form of ferric and ferrous compounds is essential for the growth and survival of gram-negative bacterial pathogens, and the ability to acquire these nutrients is considered a virulence trait (8,34). In gram-positive organisms, such as the streptococci, the role of divalent metals in virulence is less welldefined. Evidence indicates that the lipoprotein receptor-associated antigen I (LraI) family of polypeptides found in a variety of streptococci (4,7,22,23,35) and of which FimA of S. parasanguis FW213 is a member forms part of a new family of solute-binding receptors of ABC metal ion transporters. Previous studies have shown that Streptococcus gordonii (24) and Streptococcus pneumoniae (9) transporters mediate uptake of Mn 2ϩ , while recent work indicates that the Streptococcus pyogenes LraI polypeptide binds Zn 2ϩ , Cu 2ϩ , and Fe 3ϩ (21). S. parasanguis FimA is a major virulence factor associated with endocarditis, and it has been suggested that FimA functions in the development of the infection by facilitating adherence to fibrin (5). Other members of the LraI family, including PsaA of S. pneumoniae and SloC of Streptococcus mutans,...

ThesloABCROperon ofStreptococcus mutansEncodes an Mn and Fe Transport System Required for Endocarditis Virulence and Its Mn-Dependent Repressor

“…The mtsABC in S. pyogenes encodes an Fe/Zn transport system that also transports Mn (29,30). The Adc Zn transporter in S. pneumoniae was proposed to transport Mn as well (14), and observations on the FimCBA system in S. parasanguis suggest that it also transports both Fe and Mn (11,49). The regulatory mechanisms and the importance of these metal ion homeostasis systems are not yet fully elucidated.…”

Interactions of the Metalloregulatory Protein SloR from Streptococcus mutans with Its Metal Ion Effectors and DNA Binding Site

“…This organization is reminiscent of the 46-bp ScaR target sequence in S. gordonii, which includes two palindromic elements that straddle the scaC promoter (24). In fact, similar repeat sequences and promoter structures appear to be conserved in the control regions of the fim, psa, and mts operons across several streptococcal species (23)(24)(25)(26)(27)(28), suggesting that they may share similar binding mechanisms with sloA.…”

StreptococcusAdherence and Colonization

“…More recently, the PsaA protein ortholog from S. pneumoniae has been shown to bind E-cadherin (16). In addition to their putative roles in adherence, these proteins are substrate-binding components of transporters for Mn 2ϩ ions (262) and possibly Fe 3ϩ ions (430). Metal ion homeostasis in streptococci is crucial for many aspects of metabolism, particularly responses to oxidative stress (260).…”

“…Streptococci that can induce platelet activation, and the resulting thrombus formation, are believed to play a role in disseminated intravascular coagulation by causing increased platelet and clotting factor consumption (Fig. 4C) (430,446). S. mutans mutants that do not produce extracellular glucan show decreased virulence in rat models of endocarditis (414).…”