SW1 is the first filamentous phage isolated from a deep-sea environment. Nevertheless, the mechanism by which the SW1 genetic switch is controlled is largely unknown. In this study, the function of the phage-encoded FpsR protein was characterized by molecular biological and biochemical analyses. The deletion of fpsR increased the copy number of SW1 ssDNA and mRNA, indicating that FpsR functions as a repressor. In addition, transcription from the fpsR promoter was shown to be increased in an fpsR deletion mutant, suggesting self-repression by FpsR. Purified FpsR bound to four adjacent operator sites (O1–O4) embedded within the fpsA promoter and the fpsA–fpsR intergenic region. A surface plasmon resonance experiment showed that FpsR can bind to the O1–O4 operators separately and with different binding affinity, and the dissociation constants of FpsR with O2 and O3 were found to be lower at 4 °C than at 20 °C. A gel permeation chromatography assay revealed that FpsR oligomerized to form tetramers. Point mutation analysis indicated that the C-terminal domain influenced the binding affinity and regulatory function of FpsR. Collectively, these data support a model in which FpsR actively regulates phage production by interacting with the corresponding operators, thus playing a crucial role in the SW1 genetic switch.

SW1是第一个从深海环境中分离出来的丝状噬菌体。然而，SW1基因开关的控制机制在很大程度上尚不清楚。在这项研究中，通过分子生物学和生化分析来表征噬菌体编码的FpsR蛋白的功能。fpsR的缺失增加了SW1 ssDNA和mRNA的拷贝数，表明FpsR充当阻遏物。此外，在fpsR缺失突变体中，从fpsR启动子的转录显示增加，这表明FpsR可自我抑制。纯化的FpsR与嵌入fpsA启动子和fpsA-fpsR的四个相邻操作员位点（O1-O4）绑定基因间区域。表面等离振子共振实验表明，FpsR可以分别与O1-O4操纵子结合，并具有不同的结合亲和力，并且发现FpsR与O2和O3的解离常数比在20°C时低。凝胶渗透色谱分析显示FpsR寡聚形成四聚体。点突变分析表明，C末端结构域影响FpsR的结合亲和力和调节功能。这些数据共同支持了一个模型，在该模型中，FpsR通过与相应的操纵子相互作用来主动调节噬菌体的产生，从而在SW1遗传开关中起着至关重要的作用。