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In suspension-cultured cells of rice (Oryza sativa L. cv. Tainung 67), two sucrose synthase genes, RSus1 and RSus2 , were found to be differentially regulated by sugars. Exogenously provided sugars triggered marked accumulation of the RSus1 transcripts and this required de novo synthesized proteins. In addition to being regulated at the transcriptional level, other regulatory mechanisms might be involved in the sugar-modulated expression of RSus1 since the RSuS1 proteins were extremely stable in sucrose-depleted cells. Glucose analogues, 3-O -methyl glucose, 6-deoxyglucose and mannose, did not trigger the induction of RSus1 expression, suggesting that hexose uptake per se and hexokinase are not involved in the sugar-sensing pathway controlling RSus1 expression. The accumulation of RSus2 mRNA was observed regardless of the presence or absence of sugars. However, newly synthesized proteins induced by sucrose and/or starvation were involved in the regulation of RSus2 expression at transcriptional and/or post-transcriptional levels. The protein level of RSuS2 was elevated under prolonged starvation condition but a reciprocal change was observed in the sucrose-fed cell. The constant presence of RSuS1 and RSuS2 proteins in both sucrose-starved and sucrose-provided cells may allow the cells to respond to sugar availability immediately. [ABSTRACT FROM AUTHOR]

One alkaline invertase and two acid invertase activities were detected in the shoots of etiolated rice (Oryza sativa) seedlings. The alkaline invertase (AlT) was purified to homogeneity through steps of ammonium sulphate fractionation, concanavalin A-Sepharose affinity chromatography (non-retained), DEAE-Sephacel chromatography and preparative electrophoresis. The pH optimum of AIT was 7.0 and the molecular mass, determined by gel filtration, was 240 kDa. It is apparently a homotetrameric enzyme (subunit molecular mass 60 kDa). The isoelectric point was 4.4 by isoelectric focusing. The best substrate of the enzyme was sucrose, with a Km of 2.53 mM. The enzyme also hydrolysed raffinose, but not maltose or lactose, so it is a β-D-fructofuranosidase. It gave negative glycoprotein staining, Of the hydrolysis products. fructose was a competitive inhibitor and glucose was a non-competitive inhibitor. Treatment with an alkaline phosphatase could activate AIT, whereas other proteins such as BSA, concanavalin A and urease had no effect on the enzyme activity. The enzyme activity was inhibited by Tris, thiol reagents and heavy metal ions. [ABSTRACT FROM AUTHOR]

The rice sucrose synthase 1 (RSus1) gene is transcriptionally induced by sucrose, and a region within its promoter, at −1117 to −958 upstream of the transcription initiation site, was found to be essential for enhancing the sucrose-induced expression. Further dissection of this region revealed that a group of nuclear proteins interact with a 39-bp fragment named A-3-2 (−1045 to −1007). A protein that specifically and directly interacted with A-3-2 was isolated from the suspension-cultured cells of rice and was subsequently identified as a purine-rich DNA-binding protein. The amino acid sequence of this protein, OsPurα, exhibited 73% identity with the Arabidopsis Purα-1 protein, and its modeled structure resembled the structure of Pur-α in Drosophila. Recombinant OsPurα expressed and purified from Escherichia coli was demonstrated to have DNA-binding activity and to interact with A-3-2 specifically. Moreover, OsPurα was able to enhance sucrose-induced expression of the β-glucuronidase (GUS) reporter gene, which was transcriptionally fused to two copies of a DNA fragment containing A-3-2 and the cauliflower mosaic virus 35S minimal promoter, in vivo. The level of OsPurα bound to A-3-2 was higher in cells cultured in the presence of sucrose; however, the level of OsPurα mRNA in cells was not affected by sucrose. The results of this study demonstrate that OsPurα participates in the regulation of RSus1 expression in response to sucrose; nevertheless, it may require other partner proteins for full function. [ABSTRACT FROM AUTHOR]

• Bamboo is distinguished by its rapid growth. To investigate sucrose metabolism in this plant, we cloned the cDNAs encoding sucrose synthase (SuS) from Bambusa oldhamii and investigated their expression in growing shoots and leaves. • Four cDNA clones, BoSus1, BoSus2, BoSus3 and BoSus4, were isolated by screening a cDNA library from etiolated bamboo shoots. Recombinant BoSuS proteins were produced in Escherichia coli and purified by immobilized metal affinity chromatography and ultrafiltration. Semi-quantitative reverse transcriptase–polymerase chain reaction (RT-PCR) was used to determine the abundance of the transcript of each gene. • BoSus1 and BoSus3 may be duplicate or homeologous genes, the sequences of which show high identity. Similarly, BoSus2 shows high identity with BoSus4. Kinetic analysis showed that the two BoSuS isoforms of each type had similar michaelis constant ( Km) values for sucrose, but different values for UDP. The four genes were expressed in various bamboo organs but were differentially regulated. The increase in the abundance of their mRNA paralleled the growth rate of the bamboo. • The results suggest that, in bamboo, SuS is encoded by at least four genes, each with a specific role in providing substrates for the polysaccharide biosynthesis and/or energy production necessary to support the rapid growth of this species. [ABSTRACT FROM AUTHOR]