A distinct group of DNA-binding proteins called the ssDNAbinding proteins (SBP) could specifically bind ssDNA and be used in processes where the double helix is separated, including DNA replication, transcription, and recombination. Because TcaR is known as a MarR family transcription regulator that binds to specific dsDNA sequence with the winged helix-turn-helix (wHTH) DNA binding domain, the ssDNA binding ability of TcaR may not be involved in transcription. In order to clarify the role of TcaR-viral ssDNA interaction, our approach is to examine it with in vitro replication. We used single-primed M13 replication assay to measure the ability of purified TcaR protein to convert a primed single-strand M13 template to the duplex form in a manner that requires processive DNA synthesis. As seen in Figure 5A , M13-based in vitro DNA replication assay showed that the addition of TcaR protein to the reaction mixture, and incubation for up to 30 min, resulted in almost no DNA replication activity compared to controls. This indicated that TcaR markedly inhibits DNA replication and that the mechanism of inhibition, at least in part, involves interaction with viral ssDNA. These results suggest a possible role for TcaR in bacteriophage resistance.