Abstract
Sucrose synthase that catalyzes the degradation of sucrose in sink organs is activated by phosphorylation. Rice SPK, a calcium dependent protein kinase, is specifically expressed in immature seeds. Antisense SPK transformants lacked accumulation of storage substances, resulting in watery seeds. Therefore, SPK is suggested to be involved in sucrose degradation in the sink organs. In vitro experiments detected phosphorylation of the serine residue in the regulatory site of sucrose synthase by SPK, suggesting that SPK is a modulator of sucrose synthase. The Spk gene is divided into two regions, 5'-UTR on chromosome 6 and the remaining sequence on the chromosome 10. Both RNAs are separately transcribed and trans-spliced. In immature seeds, however, no trans-spliced mRNAs was detected besides much transcript from the chromosome 10. Since the trans-spliced mRNA may not allow the efficient translation, some translational regulation by 5'-UTR was suggested.
