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-binding lipoprotein ScaA was increased approximately fivefold in cells cultured in low Mn2+ medium (< 0.1 microM Mn2+), but not in iron (Fe2+/Fe3+)-limited medium, and was enhanced in the presence of human saliva or serum. mRNA analysis revealed that under low Mn2+ conditions, levels of scaCBA transcript (2.6 kb) were increased > 20-fold. The Mn2+-responsive transcriptional regulator of the sca operon was purified and characterized as a 215-amino-acid residue polypeptide, designated ScaR, with 26% identity to the Corynebacterium diphtheriae diphtheria toxin repressor (DtxR). Inactivation of scaR in S. gordonii DL1 (Challis) resulted in constitutive derepression of sca operon transcription. Expression of tpx, located immediately downstream of scaA and encoding a putative thiol peroxidase, was not subject to ScaR regulation. Purified ScaR protein bound to the scaC promoter region in vitro in the presence of Mn2+ (Kd approximately 80 nM) and, to a lesser extent, in the presence of Ni2+ or Zn2+. The metalloregulator protein binding region was localized by DNA protection analysis to a 46 bp sequence encompassing the -35 and -10 promoter signatures. This sequence was well conserved within the promoters of corresponding virulence-related permease operons in other streptococci. The results identify a new Mn2+-sensing regulator of Mn2+ transport in streptococci, important for Mn2+ homeostasis during infection of the human host.