Modulation of QscR, a quorum sensing receptor of Pseudomonas aeruginosa, by truncation of a signal binding domain

In Pseudomonas aeruginosa, a multi-host pathogen, quorum sensing (QS) plays an essential role in pathogenesis, wherein LasR, QscR and RhlR, the QS regulators, control the expression of many virulence factors. In this study, we constructed a signal-binding-domain (SBD)-deleted QscR (QscR160-237) to make a signal-independently-active form of QscR. However, QscR160-237 that has only a DNA binding domain (DBD) was not fully active. It was able to bind to the target site in a signal-independent manner, but was not able to activate transcription of the target promoter. Since QscR160-237 could interfere with binding of wild-type QscR (QscRwt) to its QscR binding site, we investigated the competition between QscR160-237 and QscRwt on the QscR binding site in vivo and in vitro. When QscRwt and QscR160-237 were independently co-expressed by two different inducers, increasing expression of QscR160-237 interfered with QscRwt activity. This was verified by a competitive gel shift assay in vitro using purified QscRwt and QscR160-237. Our results show that the SBD deletion makes QscR a partially active form that has only DNA binding ability, but it can interfere with QscRwt by competitive binding.