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SYSTEMS BIOLOGY, MOLECULAR BIOLOGY, AND GENE REGULATION

Carbon Cycling in Anabaena sp. PCC 7120. Sucrose Synthesis in the Heterocysts and Possible Role in Nitrogen Fixation^1,[OA]

Centro de Investigaciones Biológicas, Fundación para Investigaciones Biológicas Aplicadas, 7600 Mar del Plata, Argentina

Nitrogen (N) available to plants mostly originates from N₂ fixation carried out by prokaryotes. Certain cyanobacterial species contribute to this energetically expensive process related to carbon (C) metabolism. Several filamentous strains differentiate heterocysts, specialized N₂-fixing cells. To understand how C and N metabolism are regulated in photodiazotrophically grown organisms, we investigated the role of sucrose (Suc) biosynthesis in N₂ fixation in Anabaena sp. PCC 7120 (also known as Nostoc sp. PCC 7120). The presence of two Suc-phosphate synthases (SPS), SPS-A and SPS-B, directly involved in Suc synthesis with different glucosyl donor specificity, seems to be important in the N₂-fixing filament. Measurement of enzyme activity and polypeptide levels plus reverse transcription-polymerase chain reaction experiments showed that total SPS expression is greater in cells grown in N₂ versus combined N conditions. Only SPS-B, however, was seen to be active in the heterocyst, as confirmed by analysis of green fluorescent protein reporters. SPS-B gene expression is likely controlled at the transcriptional initiation level, probably in relation to a global N regulator. Metabolic control analysis indicated that the metabolism of glycogen and Suc is likely interconnected in N₂-fixing filaments. These findings suggest that N₂ fixation may be spatially compatible with Suc synthesis and support the role of the disaccharide as an intermediate in the reduced C flux in heterocyst-forming cyanobacteria.

² Present address: Pioneer Hi-Bred International (a DuPont company), 7300 N.W. 62nd Ave., Johnston, IA 50131.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Graciela L. Salerno (gsalerno{at}fiba.org.ar).

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.091736

^* Corresponding author; e-mail gsalerno{at}fiba.org.ar; fax 54–223–475–7120.

Received October 21, 2006; accepted January 8, 2006; published January 19, 2007.