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, encodin g isoform II of starch branching enzyme (SBE) from an Arabidopsis thal iana genomic library. Although a partial cDNA clone encoding the isofo rm I of the potato SEE was used as probe, no clones corresponding to t his isoform were found in the Arabidopsis library. Sbc2.1 was complete ly and Sbe2.2 partially sequenced. PCR-screening of a large number of individual plants using gene specific primers, revealed that both gene s were independently present in the genome of A. thaliana. The Sbc2.1 gene consisted of 18 exons interrupted by 17 introns and the open read ing frame comprised 2574 by encoding a protein with a calculated molec ular mass of 97660 Da. The promoter region of both Sbe2 genes containe d 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 r esponse 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 fro m both Sbe2 genes accumulated strongly when plants were incubated in l ight and with exogenous supply of either glucose, fructose or sucrose. No significant changes in Sbe2 transcript levels were detected in pla nts incubated with sorbitol under conditions of dark or light, indicat ing that the expression of Sbe2 genes is stimulated by specific carboh ydrate signals. (C) 1998 Elsevier Science Ireland Ltd. All tights rese rved.