PARENT SESSION
Posters P8C C4 and CAM. Abstracts (685-698)

Regulation of -carboxylation enzymes in marine diatoms. Espen Granum^*,1, Richard Leegood¹, John Raven², ¹ Department of Animal & Plant Sciences, Sheffield, United Kingdom² Biological Sciences Institute, Dundee, United Kingdom

ABSTRACT- Diatoms are significant primary producers, contributing at least 15 Pg C of the 50 Pg C fixed each year in the ocean. This remarkable achievement is obtained by means of efficient carbon concentrating mechanisms. Recently Reinfelder et al. (2000) found evidence of a novel unicellular C₄ pathway in a marine diatom. In this pathway -carboxylation enzymes presumably play important roles: phosphoenolpyruvate carboxylase (PEPC) in primary C fixation into C₄ acids in the cytoplasm and phosphoenolpyruvate carboxykinase (PEPCK) in decarboxylation to generate CO₂ for Rubisco in the chloroplast. However, these enzymes can also play important roles in anaplerosis and cataplerosis in relation to the Krebs cycle and amino acid metabolism. To test the C₄ pathway hypothesis we have cloned PEPC and PEPCK genes in two cosmopolitan marine diatoms, Thalassiosira pseudonana and Thalassiosira weissflogii, and measured their expression. Genomic sequences were first obtained by BLAST searching and aligning shotgun sequence data for T. pseudonana available from the US Department of Energy Joint Genome Institute. We found two different diatom PEPC genes, which are only 44% identical at amino acid level, and 36% identical to both plant (Arabidopsis thaliana) and bacterial (Escherichia coli) genes. Only one diatom PEPCK gene was found, which is 46% identical to bacterial gene and 34% to plant gene. In contrast to plant PEPC and PEPCK, N-terminal regulatory phosphorylation domains were absent in the diatom enzymes. Expression of the genes was measured under different growth conditions using RT-PCR.

KEY WORDS: phosphoenolpyruvate carboxylase, marine diatoms, phosphoenolpyruvate carboxykinase, C₄ pathway