Wolfgang Marwan

Spectroscopic Detection of a Phytochrome-like Photoreceptor in the Myxomycete Physarum polycephalum and the Kinetic Mechanism for the Photocontrol of Sporulation by Pfr¶

Abstract Sporulation of the true slime mold Physarum polycephalum (Myxomycetales) can be triggered by the far-red/red reversible Physarum phytochrome. Physarum plasmodia were analyzed with a purpose-built dual-wavelength photometer that is designed for phytochrome measurements. A photoreversible absorbance change at 670 nm was monitored after actinic red (R) and far-red (FR) irradiation of starved plasmodia, confirming the occurrence of a phytochrome-like photoreceptor in Physarum spectroscopically. These signals were not found in growing plasmodia, suggesting the Physarum phytochrome to be synthesized during starvation, which makes the cells competent for the photoinduction of sporulation. The photoconversion rates by R and FR light were similar in the phytochromes of Physarum and etiolated oat shoots. In dark-grown Physarum plasmodia that had not been preexposed to any light only R induced a detectable absorbance change while FR did not. This indicates that most (at least 90%) of the photoreversible pigment occurs in the red-absorbing form. Since the effectiveness of FR in triggering sporulation was enhanced by preirradiation with R, it is concluded that at least part of the Pr can be photoconverted to the active Pfr photoreceptor species. We propose a kinetic mechanism for the photocontrol of sporulation by photoconversion of Pfr, which may also hold for the high-irradiance response to FR in Arabidopsis and Cuscuta.

A cytoplasmic domain is required for the functional interaction of SRI and HtrI in archaeal signal transduction

Phototaxis in the archaeon Halobacterium salinarium is mediated by a stable complex of the photoreceptor sensory rhodopsin I and its transducer HtrI, which relays the light stimulus to the signalling pathway. Removal of the cytoplasmic signalling domain of HtrI eliminated the SRI‐specific motor response to light stimulation and led to the loss of the spectroscopically detectable physical interaction of SRI and HtrI. A similar phenotype was obtained by deleting part of a cytoplasmic loop located between the second transmembrane helix of HtrI and the signalling domain. These results indicate that the photochemical behavior of sensory rhodopsin I is not determined by interaction with the transmembrane helices of HtrI per se but functionally coupled to the signalling domain. It is proposed that light excitation of SRI results in a conformational change of the transducer which is conducted by the cytoplasmic loop, an extra module not found in the eubacterial transducer homologues, and activates the signalling domain.

Phytochrome-induced expression of lig1, a homologue of the fission yeast cell-cycle checkpoint gene hus1, is associated with the developmental switch in Physarum polycephalum plasmodia.

Lig1 was found in a differential-display screen for early genes expressed during phytochrome-controlled sporulation of Physarum polycephalum plasmodia. A stretch of 218 amino acids of the predicted sequence of Lig1 shares 32% sequence identity to that of the Schizosaccharomyces pombe cell-cycle and DNA-damage checkpoint gene hus1. In addition Lig1 is homologous to proteins of unknown function in Homo sapiens (35% identity) and Mus musculus (31% identity). Induction of lig1 expression was found to be controlled downstream from the point of integration of the phytochrome-activated pathway and the pathway sensing the metabolic state, but upstream of the developmental switch. The lig1 expression level in individual plasmodia correlated positively with the probability to sporulate. Sensory control of the lig1 expression level and its association with the developmental switch suggests a possible mechanism for the coordination of differentiation and the control of cell-cycle progression during the sporulation of P. polycephalum.

Signal Transduction in Halobacteria

The search for photosynthetically efficient green light and the avoidance of inefficient blue light or lethal ultra-violet light allows halobacteria to survive by means of photosynthesis in a natural habitat of brines and salt ponds under strong sunlight. The entire photobiochemistry of these archaebacteria is based upon the activation of retinal proteins. Two light-driven ion pumps, bacteriorhodopsin as a proton pump and halorhodopsin as a chloride pump, representlight energy converters that power the energy-driven processes of the cell (Lanyi, 1984). Two light sensors, sensory rhodopsin and protein P480 (also called SR-II or phoborhodopsin) mediate “colour” vision that aids the cell in finding the optimal photosynthetic environment (Spudich & Bogomolni, 1984; Takahashi et al., 1985; Marwan & Oesterhelt, 1987).

Realm of the Senses

Sensory Receptors and Signal Transduction.Edited byJohn L. Spudich Birgit H. Satir. Wiley: 1992. Pp. 269. £79,

Bacteriorhodopsin is involved in halobacterial photoreception.

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