Differential interaction of Aggregatibacter (Actinobacillus) actinomycetemcomitans LsrB and RbsB proteins with autoinducer 2

Our previous studies showed that the Aggregatibacter actinomycetemcomitans RbsB protein interacts with cognate and heterologous autoinducer 2 (AI-2) signals and suggested that the rbsDABCK operon encodes a transporter that may internalize AI-2 (D. James et al., Infect. Immun. 74:4021-4029, 2006.). However, A. actinomycetemcomitans also possesses genes related to the lsr operon of Salmonella enterica serovar Typhimurium which function to import AI-2. Here, we show that A. actinomycetemcomitans LsrB protein competitively inhibits the interaction of the Vibrio harveyi AI-2 receptor (LuxP) with AI-2 from either A. actinomycetemcomitans or V. harveyi. Interestingly, LsrB was a more potent inhibitor of LuxP interaction with AI-2 from V. harveyi whereas RbsB competed more effectively with LuxP for A. actinomycetemcomitans AI-2. Inactivation of lsrB in wild-type A. actinomycetemcomitans or in an isogenic RbsB-deficient strain reduced the rate by which intact bacteria depleted A. actinomycetemcomitans AI-2 from solution. Consistent with the results from the LuxP competition experiments, the LsrB-deficient strain depleted AI-2 to a lesser extent than the RbsB-deficient organism. Inactivation of both lsrB and rbsB virtually eliminated the ability of the organism to remove AI-2 from the extracellular environment. These results suggest that A. actinomycetemcomitans possesses two proteins that differentially interact with AI-2 and may function to inactivate or facilitate internalization of AI-2.