title: Differential Regulation of Duplicate Light-Dependent Protochlorophyllide Oxidoreductases in the Diatom Phaeodactylum tricornutum.
abstract: BACKGROUND
  Diatoms (Bacilliariophyceae) encode two light-dependent protochlorophyllide oxidoreductases (POR1 and POR2) that catalyze the penultimate step of chlorophyll biosynthesis in the light. Algae live in dynamic environments whose changing light levels induce photoacclimative metabolic shifts, including altered cellular chlorophyll levels. We hypothesized that the two POR proteins may be differentially adaptive under varying light conditions. Using the diatom Phaeodactylum tricornutum as a test system, differences in POR protein abundance and por gene expression were examined when this organism was grown on an alternating light:dark cycles at different irradiances; exposed to continuous light; and challenged by a significant decrease in light availability.
  
  
  RESULTS
  For cultures maintained on a 12h light: 12h dark photoperiod at 200μE m-2 s-1 (200L/D), both por genes were up-regulated during the light and down-regulated in the dark, though por1 transcript abundance rose and fell earlier than that of por2. Little concordance occurred between por1 mRNA and POR1 protein abundance. In contrast, por2 mRNA and POR2 protein abundances followed similar diurnal patterns. When 200L/D P. tricornutum cultures were transferred to continuous light (200L/L), the diurnal regulatory pattern of por1 mRNA abundance but not of por2 was disrupted, and POR1 but not POR2 protein abundance dropped steeply. Under 1200μE m-2 s-1 (1200L/D), both por1 mRNA and POR1 protein abundance displayed diurnal oscillations. A compromised diel por2 mRNA response under 1200L/D did not impact the oscillation in POR2 abundance. When cells grown at 1200L/D were then shifted to 50μE m-2 s-1 (50L/D), por1 and por2 mRNA levels decreased swiftly but briefly upon light reduction. Thereafter, POR1 but not POR2 protein levels rose significantly in response to this light stepdown.
  
  
  CONCLUSIONS
  Given the sensitivity of diatom por1/POR1 to real-time light cues and adherence of por2/POR2 regulation to the diurnal cycle, we suggest that POR1 supports photoacclimation, whereas POR2 is the workhorse for daily chlorophyll synthesis.
authors.affiliation: Department of Biology, University of Washington, Seattle, Washington, United States of America.
authors.forename: Heather M
authors.initials: HM
authors.lastname: Hunsperger
authors.affiliation: Department of Biology, University of Washington, Seattle, Washington, United States of America.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Ford
authors.affiliation: Department of Biology, University of Washington, Seattle, Washington, United States of America.
authors.forename: James S
authors.initials: JS
authors.lastname: Miller
authors.affiliation: Department of Biology, University of Washington, Seattle, Washington, United States of America.
authors.forename: Rose Ann
authors.initials: RA
authors.lastname: Cattolico
mesh_terms: D000064:Acclimatization / Q000528:radiation effects; D000595:Amino Acid Sequence; D049109:Cell Proliferation / Q000528:radiation effects; D002734:Chlorophyll / Q000096:biosynthesis; D003624:Darkness; D017377:Diatoms / Q000166:cytology / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects*; D019143:Evolution, Molecular; D015971:Gene Expression Regulation, Enzymologic / Q000528:radiation effects*; D008027:Light; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D014158:Transcription, Genetic / Q000528:radiation effects
chemical_list: D002734:Chlorophyll; D044925:Oxidoreductases Acting on CH-CH Group Donors; C023886:protochlorophyllide reductase