SummaryCarbon metabolites suppress the expression of a-amylase genes in germinating seeds and in suspension-cultured cells of rice. We have used suspension cell culture as a model system to study the mechanisms of metabolic regulation of a-amylase gene expression in rice. Both transcription rate and mRNA stability increased as cells were starved of sucrose; the transcription rate of a-amylase genes in cells starved of sucrose for 24 h was seven times greater than in cells provided with sucrose. The half-life of a-amylase mRNA was less than 1 h in cells provided with sucrose, but increased to 12 h in cells starved of this sugar. A protein synthesis inhibitor, cycloheximide (CHX), induced massive accumulation of a-amylase mRNA in cells provided with sucrose. The longer half-life of mRNA induced by sucrose starvation and the massive accumulation of mRNA caused by CHX were specific to the a-amylase genes, since actin genes were not similarly affected. Our findings suggest that both transcriptional and post-transcriptional control mechanisms are important in the metabolic regulation of a-amylase gene expression and de n o w synthesized proteins are involved in these mechanisms. The expression of a-amylase and actin genes is regulated in an opposite manner by sugars, which also suggests the operation of a differential regulatory mechanism under different growth conditions.

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