CARBOHYDRATE STARVATION STIMULATES DIFFERENTIAL EXPRESSION OF RICE ALPHA-AMYLASE GENES THAT IS MODULATED THROUGH COMPLICATED TRANSCRIPTIONAL AND POSTTRANSCRIPTIONAL PROCESSES

Expression of alpha-amylase genes in cultured rice suspension cells is induced by sucrose starvation, To study the mechanism of sugar metabo lite regulation on the expression of individual alpha-amylase genes, D NA fragments specific to each of eight rice alpha-amylase genes were s ynthesized and used as gene-specific probes, Comparison of the relativ e abundance of mRNA revealed that expression of the eight alpha-amylas e genes in rice cells was differentially regulated by sucrose starvati on. Accumulation of all the alpha-amylase mRNAs increased in response to sucrose starvation; however, levels of the alpha Amy3 and alpha Amy 8 mRNAs were distinctly higher and constituted 90% of total alpha-amyl ase mRNAs, RNA gel blot and nuclear run-on transcription analyses demo nstrated a positive correlation between the increased transcription ra tes and the elevated steady-state levels of alpha-amylase mRNAs induce d by sucrose starvation, The half-lives of alpha Amy3, alpha Amy7, and alpha Amy8 were prolonged by sucrose-starvation; however, the stabili ty of the three mRNAs seems controlled by different mechanisms, The tr anslation inhibitors cycloheximide and anisomycin preferentially block ed the sucrose-suppressed expression of alpha Amy3 but mot that of alp ha Amy7 and alpha Amy8. These inhibitors also enhanced the sucrose sta rvation-induced accumulation of alpha Amy3 mRNA but not that of alpha Amy7 or alpha Amy8 mRNAs. Cycloheximide did mot significantly alter th e transcription rates of alpha-amylase genes, suggesting that labile p roteins may selectively stabilize the alpha Amy7 and alpha Amy8 mRNAs but destabilize the alpha Amy3 mRNA.