Expression of the virulence-related Sea (Mn2+) permease in Streptococcus gordonii is regulated by a diphtheria toxin metallorepressor-like protein ScaR

The acquisition of transition metal ions by pathogenic bacteria is crucial to their growth and survival within the human host, however, the mechanisms of metal ion homeostasis in streptococci are unknown. The scaCBA operon in the human oral bacterium Streptococcus gordonii encodes the components of an ABC-type transporter for manganese (Mn2+). Production of substrate-bindi ng lipoprotein ScaA was increased approximately fivefold in cells cultured in low Mn2+ medium (<0.1 mu M Mn2(+)), but not in iron (Fe2+/Fe3+)-limited medium, and was enhanced in the presence of human saliva or serum. mRNA ana lysis revealed that under low Mn2+ conditions, levels of scaCBA transcript (2.6 kb) were increased >20-fold. The Mn2+-responsive transcriptional regul ator of the sea operon was purified and characterized as a 215-amino-acid r esidue polypeptide, designated ScaR, with 26% identity to the Corynebacteri um diphtheriae diphtheria toxin repressor (DtxR). Inactivation of scaR in S . gordonii DL1 (Challis) resulted in constitutive derepression of sea opero n transcription. Expression of tpx, located immediately downstream of scaA and encoding a putative thiol peroxidase, was not subject to ScaR regulatio n. Purified ScaR protein bound to the scaC promoter region in vitro in the presence of Mn2+ (K-d similar to 80 nM) and, to a lesser extent, in the pre sence of Ni2+ or Zn2+. The metalloregulator protein binding region was loca lized by DNA protection analysis to a 46 bp sequence encompassing the -35 a nd -10 promoter signatures. This sequence was well conserved within the pro moters of corresponding virulence-related permease operons in other strepto cocci. The results identify a new Mn2+-sensing regulator of Mn2+ transport in streptococci, important for Mn2+ homeostasis during infection of the hum an host.