It has long been assumed that the nematode C. elegans lacks the sense of light, mainly because it lives in the dark soil and does not have eyes. However, we and others have reported the surprising observation that C. elegans in fact possesses a simple visual system; and short-wavelength light, especially UV light, induces avoidance behavior mediated by a group of photoreceptor neurons which require the function of the lite-1 gene, a member of the invertebrate taste receptor family[Ward.(2008),Liu.(2010),Edwards.(2008)]. Interestingly, a Drosophila homolog of lite-1 has also been suggested to mediate light-induced avoidance behavior in fly larva[Xiang.(2010)]. As such, lite-1 has been suggested to encode a light-sensing protein rather than act as a taste receptor.
However, more recent work has demonstrated that light, particularly UV light, generates reactive oxygen species (ROS) such as H2O2[Bhatla.(2015)]. H2O2 also induces avoidance behavior in a way similar to UV light, suggesting that light-induced avoidance behavior could be mediated by ROS produced by light illumination rather than light itself. This raises the possibility that LITE-1 functions as a chemoreceptor instead of a photoreceptor protein.
To directly test whether LITE-1 can function as a photoreceptor protein, we biochemically purified LITE-1 to homogeneity from worm lysate. We found that LITE-1 directly absorbs UV light with an extinction coefficient in the range of 10*6. This indicates that LITE-1 is 10-100 times more sensitive to light than all known photoreceptor proteins, such as rhodopsin, cryptochrome, phytochrome and phototropin. In a genetic screen, we identified missense mutations in the lite-1 gene, which disrupt LITE-1 function in vivo. By purifying LITE-1 protein harboring these mutations, we found that the mutations also abolish light-absorption by LITE-1 protein in vitro, providing insights into the biophysical mechanism underlying light-sensing by LITE-1. Our results show that LITE-1 can function as a bona fide photoreceptor and represents the most sensitive light-detecting protein identified to date. .
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