Negative phototaxis

The action spectmm of positive and negative phototaxis of Anabaena variabilis was measured recently106). This species contains no C-phycoerythrin. Accordingly, maximum activity is found at around 615 nm (Fig. 7). In addition, in this form a second maximum occurs at around 675 nm, and a third small, but distinct, one at 440 nm, both indicating that chlorophyll a is also involved in the active light absorption (see above). The utilization via photosynthesis, however, could be excluded in this case, since the trichomes oriented themselves perfectly well to the light direction in the presence of photosynthetic inhibitors, such as DCMU and DBMIB, at concentrations in which the photosynthetic oxygen evolution was almost completely inhibited. 

The situation is further complicated by the finding that in negative phototaxis, radiation between 500 and 560 nm and above 700 nm is effective in addition to the wavelengths, which are active in the positive response. Nultsch et al.106) have discus-... 

Morphological abnormalities, both positive and negative phototaxis suppressed 29... 

From such a viewpoint, we are examining primary processes of photoreactions of PYP [1] which functions as a blue light photoreceptor for a negative phototaxis of the purple sulfur bacterium Ectothiorhodospira halophila, some FP s [2] and Rh [3] by means of the fs fluorescence up-conversion measurements. In this article, we will discuss our latest results of fs fluorescence dynamics studies on PYP, because PYP is very stable for repeated irradiation which induces photocycles so that the very accurate experimental results can be obtained rather easily and also the preparation of the site-directed mutants as well as the PYP analogues with modified chromophores are rather easy. However, before that, we will summarize briefly results of our previous investigations. 

The Photoactive Yellow Protein (PYP) is the blue-light photoreceptor that presumably mediates negative phototaxis of the purple bacterium Halorhodospira halophila [1]. Its chromophore is the deprotonated trans-p-coumaric acid covalently linked, via a thioester bond, to the unique cystein residue of the protein. Like for rhodopsins, the trans to cis isomerization of the chromophore was shown to be the first overall step of the PYP photocycle, but the reaction path that leads to the formation of the cis isomer is not clear yet (for review see [2]). From time-resolved spectroscopy measurements on native PYP in solution, it came out that the excited-state deactivation involves a series of fast events on the subpicosecond and picosecond timescales correlated to the chromophore reconfiguration [3-7]. On the other hand, chromophore H-bonding to the nearest amino acids was shown to play a key role in the trans excited state decay kinetics [3,8]. In an attempt to evaluate further the role of the mesoscopic environment in the photophysics of PYP, we made a comparative study of the native and denatured PYP. The excited-state relaxation path and kinetics were monitored by subpicosecond time-resolved absorption and gain spectroscopy. 

The Photoactive Yellow Protein (PYP) is thought to be the photoreceptor responsible for the negative phototaxis of the bacterium Halorhodospira halophila [1]. Its chromophore, the deprotonated 4-hydroxycinnamic (or p-coumaric) acid, is covalently linked to the side chain of the Cys69 residue by a thioester bond. Trans-cis photoisomerization of the chromophore was proved to occur during the early steps of the PYP photocycle. Nevertheless, the reaction pathway leading to the cis isomer is still discussed (for a review, see ref. [2]). Time-resolved spectroscopy showed that it involves subpicosecond and picosecond components [3-7], some of which could correspond to a flipping motion of the chromophore carbonyl group [8,9]. 

This chapter first describes the underwater UV environment. The different types of phototactic responses, such as positive versus negative phototaxis, are then described and related to UV tolerance as well as UV vision. Finally, implications for behavioral responses to UVR are addressed, including the role of UVR in diel vertical migration (DVM) and predator-prey interactions. 

N.L. Adams (2001). UV radiation evokes negative phototaxis and covering behavior in the sea urchin Strongylocentrotus droebachiensis. Mar. Ecol. Progr. Ser., 213, 87-95. 

Fig. 1. Summary of avaiiabie knowiedge on the phototaxis signaiing pathways in H. sallnarum, R. sphaeroides, and H. halophila in a Che-iike reaction scheme. H. sali-narum contains the photoreceptors SRi and SRii, which are compiexed in the membrane to their signal transducers Htri and Htrii. These transducers modulate the autokinase activity of CheA and thus modulate the phosphorylation status of CheY. Phototaxis of R. sphaeroides proceeds via its photosynthetic reaction center (RC) and electron transfer chain (ETC) via a putative redox sensor. Positive phototaxis in H. halophila occurs via a similar pathway, while its negative phototaxis is triggered by photoactive yellow protein (PYP). The signal transduction pathway for PYP is unknown one candidate is the Che system. Possible adaptation mechanisms have been omitted from this figure.

The positive phototaxis response is triggered through the photosynthetic machinery, as is the case for Rb. sphaeroides. The wavelength dependence of the negative phototaxis response has indicated that photoactive yellow protein (PYP) functions as the dedicated photoreceptor for this response. While the mechanism for the light activation of purified PYP has been unraveled in... 

H. salinarum, the blue-shifted 83 3 intermediate, which is formed by attractant orange light, is the photoreceptor for a negative phototaxis response to near-UV light. Thus, the photoresponse toward near-UV light is only observed when the cells are simultaneously illuminated with light that initiates the photocycle of SRI (64). These spectral complications need to be carefully taken into account when designing experiments. 

Further studies showed that the same procedure also blocked both positive and negative phototaxis, but not step-down photophobic responses (unpublished results), indicating that the latter is controlled by a different photoreceptor. Interestingly, higher plants also use photoreceptors, phototropins, with a flavin as a chromophoric group (85). 

Diehn, B., Fonseca, J. R., and Jahn, T. R. (1975) High speed cinemicrography of the direct photophobic response of Eu lena and the mechanism of negative phototaxis. J. Protozool. 22,492 94. 

Rhodopsins are not the only photoreactive proteins. Another example is the photoactive yellow protein (PYP) (Figure 6.4), a small water-soluble photoreceptor responsible for the negative phototaxis of Halorhodospira halophila. This protein contains p-cou-maric acid (pCA) as a chromophore, which undergoes an ultrafast photoisomerization reaction immediately after light absorption. The photocycle involving different sequential intermediates, pG, pR, and pB, is then triggered by this photoreaction. 

Halldal, P., Ultraviolet action spectra of positive and negative phototaxis in Platymonas subcordi-formis, Physiol. Plant, 14, 133, 1961. 

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