We identified three essential conserved residues (H204, Y236 and C266) that are critical for the assembly of type 1 fimbriae in this organism. rapid

amplification of cDNA ends analyses and reverse transcriptase-PCR results indicate that srtC1 was transcribed together with the putative adhesin gene fimQ and major structural subunit gene fimP as a single polycistronic mRNA. Actinomyces oris T14V (Henssge et al., 2009), formerly known as Actinomyces naeslundii T14V, a member of A. naeslundii genospecies 2 family, is considered as one of the primary colonizers for the formation of dental plaque on tooth surfaces (Li et al., 2004). Actinomyces oris T14V possesses two immunologically distinct types of fimbriae, which mediate the attachment of this species to both hard and soft HER2 inhibitor tissue surfaces (Cisar et al., 1988). These fimbriae were among one of the first observed in gram-positive bacteria (Girard & Jacius, 1974). Type 1 fimbriae promote the binding of this organism to tooth surfaces mediated

by the adsorbed salivary acidic proline-rich proteins and statherin. These salivary proteins serve as receptors for type 1 fimbriae (Clark VE-821 mouse et al., 1984; Gibbons et al., 1988). Type 2 fimbriae mediate the adherence of A. oris to oral mucosal epithelial cells and lactose-sensitive coaggregations with certain oral streptococci. Such interactions with other bacteria further promote the formation of dental plaque initiated by type 1 fimbriae of the organism (Palmer et al., 2003). Previously, we demonstrated that the biogenesis of functional type 1 fimbriae in A. oris T14V required three genes (Yeung et al., 1987; Chen et al., 2007): the putative adhesin gene fimQ, the major structural subunit gene fimP and the type 1 fimbria-specific sortase gene srtC1. Sequence alignment indicates that A. oris SrtC1 contains Cell press all three conserved domains (D1, D2 and D3) that are present in all sortases and an extra C-terminal hydrophobic domain. According to the sortase classification (Dramsi et al.,

2005), SrtC1 belongs to class C sortase family. Sortases are a group of bacterial thiol transpeptidases responsible for the covalent attachment of specific surface proteins to the cell wall envelope of gram-positive bacteria (Marraffini et al., 2006). These enzymes are involved in the expression of several virulence factors and the assembly of fimbriae, and have been considered as a target of anti-infective therapy (Maresso & Schneewind, 2008). SrtC1 is required for both the assembly of type 1 fimbriae in A. oris T14V and its adherence to saliva-coated hydroxylapatite (Chen et al., 2007). Accordingly, preventing the formation of type 1 fimbriae in A. oris by inhibiting the function of this sortase may reduce the colonization of this organism and consequently the dental plaque formation.