title: Carbon cycling in Anabaena sp. PCC 7120. Sucrose synthesis in the heterocysts and possible role in nitrogen fixation.
abstract: Nitrogen (N) available to plants mostly originates from N(2) 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 N(2)-fixing cells. To understand how C and N metabolism are regulated in photodiazotrophically grown organisms, we investigated the role of sucrose (Suc) biosynthesis in N(2) 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 N(2)-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 N(2) 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 N(2)-fixing filaments. These findings suggest that N(2) 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.
authors.affiliation: Centro de Investigaciones Biológicas, Fundación para Investigaciones Biológicas Aplicadas, 7600 Mar del Plata, Argentina.
authors.forename: Andrea C
authors.initials: AC
authors.lastname: Cumino
authors.forename: Clarisa
authors.initials: C
authors.lastname: Marcozzi
authors.forename: Roberto
authors.initials: R
authors.lastname: Barreiro
authors.forename: Graciela L
authors.initials: GL
authors.lastname: Salerno
mesh_terms: D000595:Amino Acid Sequence; D017033:Anabaena / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D001483:Base Sequence; D002244:Carbon / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D005964:Glucosyltransferases / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D049452:Green Fluorescent Proteins / Q000032:analysis; D008969:Molecular Sequence Data; D009586:Nitrogen Fixation; D011401:Promoter Regions, Genetic; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013395:Sucrose / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D049452:Green Fluorescent Proteins; D013395:Sucrose; D002244:Carbon; D005964:Glucosyltransferases; C022698:sucrose-phosphate synthase