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Published on January 19, 2007; 10.1104/pp.106.091736

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Received October 21, 2006
Accepted January 8, 2007

Carbon Cycling in Anabaena sp. PCC 7120. Sucrose Synthesis in the Heterocysts and Possible Role in Nitrogen Fixation

Andrea C. Cumino , Clarisa Marcozzi , Roberto Barreiro , and Graciela L. Salerno *

Centro de Investigaciones Biológicas (CIB), Fundación para Investigaciones Biológicas Aplicadas (FIBA), Vieytes 3103, C.C. 1348, 7600 Mar del Plata, Argentina

* Corresponding author; email: gsalerno{at}fiba.org.ar.

Nitrogen available to plants mostly originates from N2 fixation carried out by prokaryotes. Certain cyanobacterial species contribute to this energetically expensive process related to carbon metabolism. Several filamentous strains differentiate heterocysts, specialized N2-fixing cells. To understand how carbon and nitrogen metabolism are regulated in photo-diazotrophically grown organisms, we investigated the role of sucrose biosynthesis in N2 fixation in Anabaena sp. PCC 7120 (also known as Nostoc sp. PCC 7120). The presence of two sucrose-phosphate synthases, 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 RT-PCR 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 gfp-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 sucrose are likely interconnected in N2-fixing filaments. These findings suggest that N2 fixation may be spatially compatible with sucrose synthesis and support the role of the disaccharide as an intermediate in the reduced carbon flux in heterocyst-forming cyanobacteria.






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