ИСТИНА

Recently, we discovered the gene that encodes the Kunitz peptidase inhibitor-like protein (KPILP) in plants of the Solanaceae family (which includes the potato, tomato, and tobacco), and that there is little mRNA in the leaves of intact plants (unlike what is observed in the roots). However, mRNA content was sharply increased after abiotic and biotic stress. Unlike the genes that encoding the Kunitz peptidase inhibitor (KPI) in various animals, KPILP has no introns, which thereby excludes the possibility that its expression is regulated via alternative splicing. Here, we investigated the mechanism underlying the regulation of KPILP in stress conditions and verified our hypothetical model, which proposes that in an intact leaf, the ribosome “ignores” the start codon of the KPILP mRNA because it is within an unfavourable nucleotide context, and instead it prefers to translate the aORF via the mechanism of internal translation initiation. Under stress conditions, aORF translation is suppressed, and translation using the start codon of the maternal mRNA is resumed, which results in an increase in the level of mRNA content in the leaf. We tested this model on a series of vector constructs that we used to alter the nucleotide context of the start codon of the maternal mRNA, the nucleotide sequence preceding the aORF, the aORF start codon and the mRNA sequence. Analysis of the level of accumulation of mRNA encoding the mutant KPILP gene confirmed our proposed model, which states that the synthesis of the aORF product determines the degree of mRNA accumulation in the leaf. If the scanning ribosome “prefers” the start codon of the maternal mRNA (due to a favourable context or stress), then the aORF product will not be synthesized, and, therefore, there will be a high level of maternal mRNA in the leaf.