Research Notes

It may not have eyes, but the saltwater Natronobacterium pharaonis has a primitive form of vision that uses blue-light-absorbing sensory rhodopsin II proteins (SRII) embedded in its membrane bilayer. When activated, SRII sends signals that are "translated into flagellar motion," says Harmut "Hudel" Luecke, professor of molecular biology and biochemistry, University of California, Irvine. SRII's signaling enables bacteria to swim away from harsh sunlit areas where blue light would otherwise cause significant damage. Until recently, scientists have understood little about how SRII functions, or even what it looks like. Last month, Luecke and his UC-Irvine colleagues and others at the University of Texas Medical School reported the first high-resolution crystal structure of SRII (Science Express: www.sciencemag.org/cgi/expresspdf/1062977v1.pdf). SRII's peculiar aspect, says Luecke, is that even though its chromophore--a vitamin A derivative--is chemically identical to that of other rhodopins, the latter absorb green-orange light at 570 to 590 nm, whereas SRII absorbs the more intense blue light, at 497 nm. The researchers found that the main reason for this blue-light shift is a 1.1 Å repositioning of a charged Arginine residue (Arg⁷²). Surprisingly, Arg⁷² is located 10 to 11 Å from where the light enters, some distance in atomic terms, says Luecke. The researchers also found "a residue on the surface [of SRII] which we believe probably interacts with the transducer HtrII," says Luecke. "Now we're working toward an X-ray structure of this complex that would clearly show how they interact."

The plant hormone abscisic acid (ABA) plays an essential role in many physiological plant processes, including how a plant responds to specific environmental stresses. Led by Jian-Kang Zhu, a research group at the University of Arizona, Tuscon, provided the first results that reveal genetic evidence indicating that phosphoinositols mediate ABA and stress-signal transduction in plants. (L. Xiong et al., "FIERY1 encoding an inositol polyphosphate 1-phosphatase is a negative regulator of abscisic acid and stress signaling in Arabidopsis," Genes and Development, 15[15]:1971-84, Aug. 1, 2001.) Zhu and his colleagues engineered Arabidopsis plants to emit bioluminescence in response to ABA treatment or an environmental stress. Researchers discovered an Arabidopsis mutation, fiery1 (FRY1), which they found to increase the expression of stress-responsive genes, rendering it more sensitive to ABA as well as other environmental stresses, like cold, drought, or salt. By mapping out the FRY1 genome, researchers revealed that the mutation encodes an inositol polyphosphate 1-phosphatase that is responsible for catabolizing IP3, suggesting that this second messenger functions in both ABA and environmental stress signaling pathways. The pathway, which is similar to the one found in animal systems, begins when phospholipases break down phosphotidylinositol 4, 5-biphosphate (PIP2), generating IP3, which then contributes in ABA responses. While Zhu wouldn't speculate on the study's immediate effects, he predicts it will provide a better understanding of how plants respond to stress, eventually leading researchers to make genetic changes that could improve long-term plant performance.