K-096. Light-Dependent Regulation of Cellular Morphology in the Cyanobacterium Fremyella diplosiphon

J. R. Bordowitz, M. A. TerAvest, B. L. Montgomery;
Michigan State Univ., East Lansing, MI.

Fremyella diplosiphon, a freshwater filamentous cyanobacterium, possesses the ability to sense and adapt to changes in ambient light. In a process called complementary chromatic adaptation (CCA), the cyanobacterium enhances its photosynthesis by altering the phycobiliprotein composition of its light-harvesting antennae. This process requires RcaE, a phytochrome-class photoreceptor, in order to occur2. Over 30 years ago, morphological differences were observed in the wild-type UTEX481 strain in response to red and green light1. Therefore, we hypothesize that RcaE, which has previously been determined to control pigmentation in response to light3, regulates the observed light-dependent morphological changes. We have characterized this photomorphogenic response using confocal microscopy for observation of cellular morphology and visualization of phycobiliprotein autofluorescence and biochemical quantification of phycobiliproteins using both fluorescence- and absorbance-based methods. Our results show that both UTEX481 and SF33, a shortened-filament strain with normal CCA pigmentation responses, maintain distinct RL and GL morphologies. Additional analyses of an RcaE null mutant strain (FdBk14) indicate that RcaE regulates cell shape in F. diplosiphon in response to red and green light. Phycobiliprotein quantification and the round, bubble-like cellular morphology of the mutant suggests that this difference may be due to changes in cell wall structure and/or permeability. Furthermore, detailed light-switching experiments indicate that RcaE regulation of light-dependent morphology is photoreversible and, at least for RL, is mediated via the Rca signaling system. 1.Bennett, A., and Bogorad, L. (1973) Complementary chromatic adaptation in a filamentous blue-green alga. J Cell Biol 58: 419-435.2. Kehoe, D.M., and Grossman, A.R. (1996) Similarity of a chromatic adaptation sensor to phytochrome and ethylene receptors. Science 273: 1409-1412.3. Terauchi, K., Montgomery, B.L., Grossman, A.R., Lagarias, J.C., and Kehoe, D.M. (2004) RcaE is a complementary chromatic adaptation photoreceptor required for green and red light responsiveness. Mol Microbiol 51: 567-577.