Color-sensitive motility and methanol release responses in Rhodobacter sphaeroides

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 v olatile tritiated compounds from [methyl-H-3]methionine-labeled cells, resp ectively. Increases in the stop frequency and the rate of methanol release were induced by exposure of cells to repellent light signals, such as an in crease in blue- and a decrease in infrared-light intensity. At a lambda of >500 nm the amplitude of the methanol release response followed the absorba nce spectrum of the photosynthetic pigments, suggesting that they function as photosensors for this response. In contrast to the previously reported m otility response to a decrease in infrared light, the blue-light response r eported here does not depend on the number of photosynthetic pigments per c ell, suggesting that it is mediated by a separate sensor. Therefore, color discrimination in taxis responses in R, sphaeroides involves two photosensi ng systems: the photosynthetic pigments and an additional photosensor, resp onding to blue light, The signal generated by the former system could resul t in the migration of cells to a light climate beneficial for photosynthesi s, while the blue-light system could allow cells to avoid too-high intensit ies of (harmful) blue light.