SspA carboxyl-terminal peptide regulates adhesion expression in S. gordonii

Objectives:  Deletion of the enzyme sortase A (SrtA) results in generalized upregulation of S. gordonii adhesin genes and proteins, whereas loss of SspAB upregulates selective adhesins. We postulate a novel mechanism of regulation by a two-component system (TCS), involving intramembrane sensing of the C-terminal peptide (C-pep) of SspA produced by SrtA processing.  The purpose of this study is to verify a widely conserved but previously uncharacterized intramembrane histidine kinase (SGO_1180) predicted to signal for regulated adhesin expression.  We also seek to characterize the cystolic response regulator (SGO_1181) interacting with SGO_1180, which are proposed to signal for transcriptional regulation of adhesin gene expression in S. gordonii.

Methods:  S. gordonii mutants for both SGO_1180 and SGO_1181 were generated.  These strains, wild-type S. gordonii DL1, and other previously characterized adhesin mutant strains were analyzed in a 96-well plate biofilm formation assay. Mutant strains were inoculated into antibiotic-supplemented THB and grown for 24 hrs.  Wild-type cells were treated identically except no antibiotic was added.  Biofilm formation in each well was quantified spectrophotometrically using crystal violet stain.  

Results:  When compared to wild-type, S. gordonii SGO_1180 and SGO_1181 mutants showed increased biofilm formation, which was similar to the biofilm phenotypes of previously characterized SrtA and SspAB mutants.  

Conclusions:  SGO_1180 and SGO_1181 gain-of-function mutants appear to represent novel sensor and response regulator components, respectively, of a two-component system (TCS) involved in regulating biofilm formation.  Previous studies suggest that this TCS responds to the C-pep cleaved from SspA by SrtA.  Future studies will characterize the interactions between C-pep and SGO_1180 and SGO_1181 mutants to understand how this novel TCS affects expression of other S. gordonii LPXTG-motif adhesin proteins.