Biosynthesis of sucrose-6-P catalyzed by sucrose-phosphate synthase (SPS), and the presence of sucrosephosphate phosphatase (SPP) leading to the formation of sucrose, have both been ascertained in a prokaryotic organism: Anabaena 7119, a filamentous heterocystic cyanobacterium. Two SPS activities (SPS-I and SPS-II) were isolated by ion-exchange chromatography and partially purified. Four remarkable differences between SPSs from Anabaena and those from higher plants were shown: substrate specificity, effect of divalent cations, native molecular mass, and oligomeric composition. Both SPS-I and SPS-II accept Fru-6-P (K m for SPS-I ‫؍‬ 0.8 ؎ 0.1 mM; K m for SPS-II ‫؍‬ 0.7 ؎ 0.1 mM) and UDP-Glc as substrates (K m for SPS-I ‫؍‬ 1.3 ؎ 0.4 mM; K m for SPS-II ‫؍‬ 4.6 ؎ 0.4 mM), but unlike higher plant enzymes, they are not specific for UDP-Glc. GDP-Glc and TDP-Glc are also SPS-I substrates (K m for GDP-Glc ‫؍‬ 1.2 ؎ 0.2 mM and K m for TDP-Glc ‫؍‬ 4.0 ؎ 0.4 mM), and ADP-Glc is used by SPS-II (K m for ADP-Glc ‫؍‬ 5.7 ؎ 0.7 mM). SPS-I has an absolute dependence toward divalent metal ions (Mg 2؉ or Mn 2؉ ) for catalytic activity, not found in plants. A strikingly smaller native molecular mass (between 45 and 47 kDa) was determined by gel filtration for both SPSs, which, when submitted to SDS͞PAGE, showed a monomeric composition. Cyanobacteria are, as far as the authors know, the most primitive organisms that are able to biosynthesize sucrose as higher plants do.

The pathway of sucrose metabolism in cyanobacteria is just starting to be elucidated. The present study describes the first isolation and biochemical characterization of a prokaryotic sucrose synthase (SS, EC 2.4.1.13). Two SS forms (SS-I and SS-II) were detected in Anabaena sp. strain PCC 7119. The isoform SS-II was purified 457-fold and its amino-terminal portion sequenced. Substrate specificity, kinetic constants, native protein and subunit molecular masses, and the effect of different ions and metabolites were studied for SS-II. Anabaena SS was shown to be a tetramer with a 92-kDa polypeptide that was recognized by maize SS polyclonal antibodies. Some striking differences from plant enzymes were demonstrated with respect to substrate affinities, regulation by metal ions and ATP, and the amino-acid sequence of the N-terminal region.

Arabinoxylan arabinosyltransferase (AX-AraT) activity was investigated using microsomes and Golgi vesicles isolated from wheat (Triticum aestivum) seedlings. Incubation of microsomes with UDP-[14C]-β-l-arabinopyranose resulted in incorporation of radioactivity into two different products, although most of the radioactivity was present in xylose (Xyl), indicating a high degree of UDP-arabinose (Ara) epimerization. In isolated Golgi vesicles, the epimerization was negligible, and incubation with UDP-[14C]Ara resulted in formation of a product that could be solubilized with proteinase K. In contrast, when Golgi vesicles were incubated with UDP-[14C]Ara in the presence of unlabeled UDP-Xyl, the product obtained could be solubilized with xylanase, whereas proteinase K had no effect. Thus, the AX-AraT is dependent on the synthesis of unsubstituted xylan acting as acceptor. Further analysis of the radiolabeled product formed in the presence of unlabeled UDP-Xyl revealed that it had an apparent molecular mass of approximately 500 kD. Furthermore, the total incorporation of [14C]Ara was dependent on the time of incubation and the amount of Golgi protein used. AX-AraT activity had a pH optimum at 6, and required the presence of divalent cations, Mn2+ being the most efficient. In the absence of UDP-Xyl, a single arabinosylated protein with an apparent molecular mass of 40 kD was radiolabeled. The [14C]Ara labeling became reversible by adding unlabeled UDP-Xyl to the reaction medium. The possible role of this protein in arabinoxylan biosynthesis is discussed.

Sucrose synthase (SS), a key enzyme in plant carbohydrate metabolism, has recently been isolated from Anabaena sp. strain PCC 7119, and biochemically characterized; two forms (SS-I and SS-II) were detected (Porchia et al. 1999, Planta 210: 34-40). The present study describes the first isolation and characterization of a prokaryotic SS gene, susA, encoding SS-II from that strain of Anabaena. A 7 kbp DNA fragment containing an open reading frame (EMBL accession number AJ010639) with about 30-40% amino acid identity with plant SSs was isolated from an Anabaena subgenomic library. The putative SS gene was demonstrated to encode an SS protein by expression in Escherichia coli. The biochemical properties of the recombinant enzyme were identical to those of the enzyme purified from the cyanobacterial cells. The deduced amino acid sequence of the Anabaena SS diverged from every plant SS reported. The occurrence of SS in cyanobacteria of different taxonomic groups was investigated. The enzyme occurs in several filamentous nitrogen-fixing cyanobacteria but not in two species of unicellular, non-diazotrophic cyanobacteria.

Incorporation of [ 3 H]arabinose and [ 14 C]ferulic acid into soluble and polymeric fractions from suspension-cultured wheat (Triticum aestivum L.) cells and the corresponding extracellular medium was studied.