Abstract
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.
Highlights
Sensory rhodopsin regulates phototactic signalling proteins by interacting with a specific cognate partner transducer called the halobacterial transducer of rhodopsin, or Htr[11,12]
In 2010, we reported a unique six-rhodopsin system in Haloarcula marismortui[24] comprised of two bacteriorhodopsins, one halorhodopsin and three sensory rhodopsins[24,25]
SRM-HtrM is a member of the three-sensory rhodopsin system in H. marismortui (Fig. 1b)[24]
Summary
Sensory rhodopsin regulates phototactic signalling proteins by interacting with a specific cognate partner transducer called the halobacterial transducer of rhodopsin, or Htr[11,12]. SRI and SRII, together with their transducers HtrI and HtrII, mediate positive and negative phototaxis, respectively. We named the third sensory rhodopsin “SRM”, designating it as the middle-type between SRI and SRII In this three-sensory rhodopsin system, SRI and SRII were shown to regulate positive and negative phototaxis, respectively[8,9,10]. H. marismortui SRM-HtrM fusion protein was transformed into H. salinarum, which contains its endogenous SRI/SRII sensory rhodopsin system. Phototactic measurements showed that SRM-HtrM attenuated phototactic responses to green light mediated by both SRI and SRII in H. salinarum
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