CONTROL OF TRANSCRIPTION AND MESSENGER-RNA TURNOVER AS MECHANISMS OF METABOLIC REPRESSION OF ALPHA-AMYLASE GENE-EXPRESSION

Carbon metabolites suppress the expression of alpha-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 alpha-amylase gene expression in rice. Both transcription rate and mRNA stability increased as cells were starved of sucrose; the transcription rate of alpha-amylase genes in cells starved of sucrose for 24 h was seven times greater than in cells provided with sucrose. The half-life of alpha-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 alpha-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 alpha-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 alpha-amylase gene expression and de novo synthesized proteins are involved in these mechanisms. The expression of alpha-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.