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 heterocystforming cyanobacteria.

Requirement of Fra proteins for communication channels between cells in the filamentous nitrogen-fixing cyanobacteriumAnabaenasp. PCC 7120

“…This organism, which belongs to one of the first prokaryotic groups on earth to have evolved multicellularity, had to develop structures for intercellular communication. Intercellular communication between heterocysts and vegetative cells comprises small molecules, such as sucrose moving from vegetative cells to heterocysts (2)(3)(4)(5) and a dipeptide, β-aspartyl-arginine, moving from heterocysts to vegetative cells (6,7). The mechanism of communication between heterocysts and vegetative cells has been debated for the last 50 y.…”

Sucrose synthase activity in the sus1/sus2/sus3/sus4 Arabidopsis mutant is sufficient to support normal cellulose and starch production

“…Although UDP is generally considered to be the preferred nucleoside diphosphate for SUS, numerous studies have shown that ADP serves as an effective acceptor molecule to produce ADP-glucose (ADPG) (5)(6)(7)(8)(9)(10)(11)(12)(13)(14). SUS is highly regulated both at transcriptional and posttranslational levels (15)(16)(17)(18)(19)(20)(21), and plays a predominant role in the entry of carbon into metabolism in nonphotosynthetic cells, and in determining both sink strength and phloem loading (22)(23)(24)(25).…”

Recent progress toward understanding the role of starch biosynthetic enzymes in the cereal endosperm

“…The UDPGlc is converted to adenosine 5'-diphosphate glucose (ADPGlc) in the cytosol by the stepwise reactions of UDPGlc pyrophosphorylase and ADPGlc pyrophosphorylase (AGPase), while the conversion of fructose to ADPGlc involves hexokinase, phosphoglucoisomerase and phosphoglucomutase [11]. Although UDP is the preferred nucleoside diphosphate substrate for SuSy, ADP could also be an acceptor molecule of this sucrolytic enzyme to produce ADPGlc [12][13][14]. Cytosolic ADPGlc is then thought to be transported into the amyloplast by means of Brittle-1, a membrane protein located in the envelope membranes of amyloplasts [15], whose absence results in reduced starch content [16,17].…”

“…It starts with individual chains as the first level, which are formed by ADP-glucose through (1→4)-α-glycosidic linkages. Those chains in amylopectin have been further denoted as C chains having the reducing end, A chains (degree of polymerization, DP, [6][7][8][9][10][11][12] carrying no branches, B1 chains (DP [13][14][15][16][17][18][19][20][21][22][23][24] carrying A chains, B2 chains (DP DOI 10.1515/amylase-2017-0006 [25][26][27][28][29][30][31][32][33][34][35][36] carrying B1 chains, B3 chains (DP > 36) carrying B2 chains, and so on (for a recent review, see [2]). Although still occasionally one finds statements to the contrary, amylose is not solely linear, but contains a small but significant number of long-chain branches [3].…”