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Plant and Cell Physiology Advance Access published online on June 27, 2009
Plant and Cell Physiology, doi:10.1093/pcp/pcp095
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© The Author 2009. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Identification and Expression Analysis of Light-Dependent and Light-Independent Protochlorophyllide Oxidoreductases in the Chromatically Adapting Cyanobacterium Fremyella diplosiphon UTEX 481.

Jessica Shui1, Eileen Saunders1, Robert Needleman1, Michelle Nappi1, Joseph Cooper1, Lauren Hall1, David Kehoe2 and Emily Stowe-Evans1

1Biology Department, 701 Moore Avenue, Bucknell University, Lewisburg PA, 17837
2Department of Biology, 1001 East 3rd Street, Indiana University, Bloomington, IN 47405

Corresponding author: Dr. Emily Stowe-Evans, 701 Moore Avenue, Biology Department, Bucknell University, Lewisburg, PA 17837, 570-577-1320, fax 570-577-3537, estoweva{at}bucknell.edu


  Abstract

The cyanobacteria Fremyella diplosiphon can alternate its light harvesting pigments, a process called complimentary chromatic adaptation (CCA), allowing it to photosynthesize in green light (GL) and in fluctuating light conditions. Nevertheless, F. diplosiphon requires chlorophylls for photosynthesis under all light conditions. Two alternate enzymes catalyze the penultimate step of chlorophyll synthesis, Light-dependent protochlorophyllide oxidoreductase (LPOR) and Dark-operative protochlorophyllide Oxidoreductase (DPOR). DPOR enzymatic activity is light independent, while LPOR requires light. Therefore, we hypothesize that F. diplosiphon up regulates DPOR gene expression in GL, so that DPOR is more abundant when LPOR is less functional. We cloned the genes encoding the three subunits of DPOR, chlL, chlN, and chlB, and the LPOR gene, por, to determined abundance of the transcripts under RL, GL, and dark conditions. We found that F. diplosiphon chlL and chlN genes are transcribed as parts of a single operon, a gene structure that is conserved within cyanobacteria. Transcripts levels of all DPOR genes are up regulated approximately 2-fold in GL relative to levels in RL, whereas LPOR transcript levels are reduced in GL. Moreover, mutations in CCA regulators, RcaE and CpeR, modify DPOR and LPOR transcript levels under specific light conditions. Finally, both DPOR and LPOR transcripts are down regulated 2 to 5 fold in the dark. These results provide the first evidence that light quality and CCA affect the genetic regulation of chlorophyll biosynthesis in freshwater cyanobacteria, ecologically important photosynthetic organisms.

Keywords: chlorophyll synthesis - chromatic adaptation - Fremyella diplosiphon

(Received April 30, 2009; Accepted June 25, 2009)
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