Sugar effect on the malate synthase and isocitrate lyase gene expression at the level of mRNA stability

Regulation of malate synthase (MS) and isocitrate lyase (ICL) gene at the posttranscriptional level was studied using actinomycin D, a transcription inhibitor. Detached roots which contained an abundance of NIS and ICL transcripts due to starvation for 4 days, showed a decrease in mRNA amount of about 50% after 3 h incubation in the presence of 10 ug/mI actinomycin D. In contrast, when sucrose was added together with actinomycin D, MS and ICL mRNA decreased to a much lower level within 3 h. Therefore, NIS and ICL half lives appear much shorter in the presence of sugar. Similar results were obtained with detached cotyledons. Confirmation that actinomycin D had effectively stopped transcription was obtained by showing that it prevented light induced expression of hydroxypyruvate reductase and rubisco genes in cotyledons. Thus, NIS and ICL genes appear to be regulated by sugars at the levels of transcription and mRNA stability.