Molecular Plant Advance Access published online on October 30, 2009
Molecular Plant, doi:10.1093/mp/ssp085
Molecular Characterization of the Calvin Cycle Enzyme Phosphoribulokinase in the Stramenopile Alga Vaucheria litorea and the Plastid Hosting Mollusc Elysia chlorotica
a Department of Biochemistry, Microbiology and Molecular Biology, 5735 Hitchner Hall, University of Maine, Orono, ME 04469, USA
b Department of Biology, Texas A&M University, College Station, TX 77843, USA
c Department of Ecology, Evolution and Natural Resources, Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, USA
d School of Biology and Ecology, University of Maine, Orono, ME 04469, USA
e Green Plant Institute, #2-202 Bio Valley, Seoul National University, 103-2 Seodun, Gwonseon, Suwon, Gyeonggi 441-853, Korea
f Department of Biochemistry and Biophysics, Texas A&M University, College Station, TX 77843, USA
1 To whom correspondence should be addressed. E-mail mrumpho{at}umit.maine.edu, fax 207-581-2801, tel. 207-581-2806.
Phosphoribulokinase (PRK), a nuclear-encoded plastid-localized enzyme unique to the photosynthetic carbon reduction (Calvin) cycle, was cloned and characterized from the stramenopile alga Vaucheria litorea. This alga is the source of plastids for the mollusc (sea slug) Elysia chlorotica which enable the animal to survive for months solely by photoautotrophic CO2 fixation. The 1633-bp V. litorea prk gene was cloned and the coding region, found to be interrupted by four introns, encodes a 405-amino acid protein. This protein contains the typical bipartite target sequence expected of nuclear-encoded proteins that are directed to complex (i.e. four membrane-bound) algal plastids. De novo synthesis of PRK and enzyme activity were detected in E. chlorotica in spite of having been starved of V. litorea for several months. Unlike the algal enzyme, PRK in the sea slug did not exhibit redox regulation. Two copies of partial PRK-encoding genes were isolated from both sea slug and aposymbiotic sea slug egg DNA using PCR. Each copy contains the nucleotide region spanning exon 1 and part of exon 2 of V. litorea prk, including the bipartite targeting peptide. However, the larger prk fragment also includes intron 1. The exon and intron sequences of prk in E. chlorotica and V. litorea are nearly identical. These data suggest that PRK is differentially regulated in V. litorea and E. chlorotica and at least a portion of the V. litorea nuclear PRK gene is present in sea slugs that have been starved for several months.
Key Words: Alga • Calvin cycle • Elysia chlorotica • kleptoplast • mollusc • phosphoribulokinase • photosynthesis • plastid • redox regulation • stramenopile • symbiosis • Vaucheria litorea