Two Novel Genes Encoding SNF-1 Related Protein Kinases from Arabidopsis Thaliana: Differential Accumulation of AtSR1 and AtSR2 Transcripts in Response to Cytokinins and Sugars, and Phosphorylation of Sucrose Synthase by AtSR2

We searched for genes encoding members of the group-3 SNF1-related protein kinase (SnRK3) family in the Arabidopsis thaliana database, and seven independent sequences were identified. Transcripts of two of them were found to accumulate differentially upon treatment with light, cytokinins and sugars. Full-length cDNAs were isolated and designated as AtSR1 and AtSR2; they encode polypeptides of 442 and 429 amino acids with relative molecular masses of 50.3 kDa and 48.2 kDa, respectively. In etiolated seedlings, no transcripts of either gene were observed. However, upon exposure to light or cytokinins, transcripts of AtSR1 but not AtSR2 began to accumulate. The induction with light was greatly reduced in the presence of a cytokinin antagonist, suggesting that cytokinins are involved in light-signaling pathways. In contrast, transcription of AtSR2, but not of AtSR1, was greatly increased by sucrose, as well as glucose and fructose. AtSR2 expressed in E. coli efficiently phosphorylated sucrose synthase in the presence of manganese ions. These results suggest that, although SnRK3 proteins may generally be involved in sugar metabolism, expression of AtSR1 and AtSR2 is differentially and distinctly regulated by various external signals, and AtSR2 may function in the regulation of sucrose synthase by specific phosphorylation.