The Blue-Green Sensory Rhodopsin SRM from Haloarcula marismortui Attenuates Both Phototactic Responses Mediated by...

Haloarchaea utilize various microbial rhodopsins to harvest light energy or to mediate phototaxis in search of optimal environmental niches. To date, only the red light-sensing sensory rhodopsin I (SRI) and the blue light-sensing sensory rhodopsin II (SRII) have been shown to mediate positive and negative phototaxis, respectively. In this work, we demonstrated that a blue-green light-sensing (504 nm) sensory rhodopsin from Haloarcula marismortui, SRM, attenuated both positive and negative phototaxis through its sensing region. The H. marismortui genome encodes three sensory rhodopsins: SRI, SRII and SRM. Using spectroscopic assays, we first demonstrated the interaction between SRM and its cognate transducer, HtrM. We then transformed an SRM-HtrM fusion protein into Halobacterium salinarum, which contains only SRI and SRII, and observed that SRM-HtrM fusion protein decreased both positive and negative phototaxis of H. salinarum. Together, our results suggested a novel phototaxis signalling system in H. marismortui comprised of three sensory rhodopsins in which the phototactic response of SRI and SRII were attenuated by SRM.