Regulation of structural gene expression in tobacco.

Abstract We have measured the extent to which structural gene expression is regulated in an entire tobacco plant. Using a sensitive RNA-excess/single-copy DNA hybridization strategy (Galau et al., 1976), our studies reveal that 3.0–3.4 × 10 7 nucleotides of single-copy transcript, or 24,000–27,000 average-sized mRNAs, are present in the polysomes of each organ system (leaf, root, stem, petal, anther and ovary). These mRNA sets are transcribed individually from 4.7-5.2% of the single-copy DNA. Approximately 1 × 10 7 nucleotides of single-copy transcript, or 8000 diverse mRNAs, were shown to be shared by the polysomes of all organs. The sequence composition of the remaining polysomal mRNAs was found to be highly regulated, each organ having at least 8 × 10 6 nucleotides of mRNA complexity, or 6000 diverse gene transcripts, which are not present in the polysomes of other organs. Hybridization experiments with nuclear RNA revealed that structural genes not utilized for the production of functional mRNAs in a given were nevertheless transcribed, and were represented by transcripts in the hnRNA. This suggests that post-transcriptional selection mechanisms may play an important role in the regulation of plant gene expression. Collectively, our measurements demonstrate that at least 11% of the single-copy DNA, or 60,000 diverse structural genes, are expressed in the entire plant during the dominant phase of its life cycle. This amount of genetic information constitutes only 4.6% of the tobacco genome.