Differential accumulation of Arabidopsis thaliana Sbe2.1 and Sbe2.2 transcripts in response to light

We have isolated two closely related genes, Sbe2.1 and Sbe2.2, encoding isoform II of starch branching enzyme (SBE) from an Arabidopsis thaliana genomic library. Although a partial cDNA clone encoding the isoform I of the potato SEE was used as probe, no clones corresponding to this isoform were found in the Arabidopsis library. Sbc2.1 was completely and Sbe2.2 partially sequenced. PCR-screening of a large number of individual plants using gene specific primers, revealed that both genes were independently present in the genome of A. thaliana. The Sbc2.1 gene consisted of 18 exons interrupted by 17 introns and the open reading frame comprised 2574 by encoding a protein with a calculated molecular mass of 97660 Da. The promoter region of both Sbe2 genes contained potential sucrose responsive elements (SURE1 and SURE2). In addition, a potential light responsive element (G-box) was present in the Sbc2.1 promoter. Northern blot analysis using gene specific probes, showed a differential accumulation of transcripts from the Sbe2 genes as a response to light. While the level of Sbe2.1 transcripts increased and decreased during light and dark, respectively, those transcribed from the Sbe2.2 gene did not change significantly. However, transcripts from both Sbe2 genes accumulated strongly when plants were incubated in light and with exogenous supply of either glucose, fructose or sucrose. No significant changes in Sbe2 transcript levels were detected in plants incubated with sorbitol under conditions of dark or light, indicating that the expression of Sbe2 genes is stimulated by specific carbohydrate signals. (C) 1998 Elsevier Science Ireland Ltd. All tights reserved.