Color-Sensitive Motility and Methanol Release Responses in Rhodobacter sphaeroides

Abstract

Blue-light-induced repellent and demethylation responses, characteristic of behavioral adaptation, were observed in Rhodobacter sphaeroides. They were analyzed by computer-assisted motion analysis and through the release of volatile tritiated compounds from [methyl-3 H]methionine-labeled cells, respectively. Increases in the stop frequency and the rate of methanol release were induced by exposure of cells to repellent light signals, such as an increase in blue-and a decrease in infrared-light intensity. At a of >500 nm the amplitude of the methanol release response followed the absorbance spectrum of the photosynthetic pigments, suggesting that they function as photosensors for this response. In contrast to the previously reported motility response to a decrease in infrared light, the blue-light response reported here does not depend on the number of photosynthetic pigments per cell, suggesting that it is mediated by a separate sensor. Therefore, color discrimination in taxis responses in R. sphaeroides involves two photosensing systems: the photosynthetic pigments and an additional photosensor, responding to blue light. The signal generated by the former system could result in the migration of cells to a light climate beneficial for photosynthesis, while the blue-light system could allow cells to avoid too-high intensities of (harmful) blue light.Changes in the environmental light climate induce behavioral responses in photosynthetic purple bacteria, which allow them to migrate towards an environment beneficial for photosynthesis. Reports on photosynthetic bacteria that accumulate in infrared light are among the classics of microbiology (2, 3). In more recent years, more-complex behavioral responses of these organisms towards light have been reported. Free-swimming Ectothiorhodospira halophila cells, in addition, show a repellent response, with a maximal relative increase in the flagellar reversal frequency at 450 nm (20). A colony of Rhodospirillum centenum is repelled by intense green light (550 to 600 nm) and is attracted by red or infrared light that is absorbed by the photosynthetic pigments (15). Light-induced motility responses have also been analyzed in Rhodobacter sphaeroides WS8-N (1). This bacterium responds to a step-down in yellow-green light (530 to 600 nm), and infrared light in a background of red monitoring light (650 Ϯ 10 nm), by an increase in its stop or reorientation frequency, with adaptation taking approximately 2 min. The photosynthetic apparatus is the photosensor for this response (6).Stimulus-induced turnover of methylated carboxyl groups of methyl-accepting transducer proteins is a common feature of prokaryotic taxis-signaling pathways. Nevertheless, previous methanol release assays with intact R. sphaeroides cells have not revealed changes in methanol release upon the addition or removal of chemoeffectors (19). The recent finding of chemotaxis operons encoding MCP homologues, methyltransferases and -esterases, however, supports the involvement of a methylation-dependent adaptation p...

Photoactive Yellow Protein, the Xanthopsin

“…It is relevant to note that this does not hold for the xanthopsin from Rb. sphaeroides, provided that no genetic redundancy exists in this organism (Kort et al 2000). The results of the sequence comparison shown in Figure 19.1, revealing three sub-groups, are consistent with this conclusion.…”

CheR- and CheB-Dependent Chemosensory Adaptation System of Rhodobacter sphaeroides

“…A modified version of the methanol release assay developed for R. sphaeroides was used (21). All experiments were carried out under photoheterotrophic conditions.…”

Identification of Six New Photoactive Yellow Proteins—Diversity and Structure–Function Relationships in a Bacterial Blue Light Photoreceptor†

“…The pB lifetime of the PYPs from the first group is suitable for a role in phototaxis, as supported for H. halophila (42); that of the PYPs from the third group matches a role in gene expression, as found for R. centenum (9). The functional role for the PYPs with a surprisingly rapid photocycle remains unknown (see Kort et al [72]).…”

“…The functional role for the PYPs with a surprisingly rapid photocycle remains unknown (see Kort et al . [72]).…”