ИСТИНА

Short open reading frames (sORFs) are a common feature of all genomes, but few functional peptides encoded by out-of-frame overlapping sORFs are known. Recently, we identified and isolated the leaf necrosis associated factor (LNAF) gene from Nicotiana species. The LNAF mRNA level is very low in intact N. tabacum and N. benthamiana leaves, but it is drastically increased in plant leaves with necrosis caused by Tobacco mosaic virus infection, viral-vector-directed protein overproduction,and chloroplast dysfunction. In contrast to intact leaves, the levels of LNAF and its mRNA in the roots are high and increased with the age of the plant. To gain a clearer understanding of the mechanism of LNAF gene regulation, we identified a sORF in the LNAF mRNA that encodes a 53-aa peptide (53-Pep). The 53-Pep ORF has an out-of-LNAF-frame start codon in an optimal context and ends with a stop codon within the LNAF gene. We showed that 53-Pep fused with the GFP or GUS is detected in transfected N. benthamiana leaves. Mutagenesis of the AUG start codon of 53-Pep sORF in the LNAF mRNA sequence increased LNAF protein production levels and the accumulation of the corresponding mRNA by 8-10-fold in the plant cells. We concluded that 53-Pep sORF is regulated by chloroplasts and responsible for constraining the expression of the maternal LNAF gene in intact leaves. 53-Pep sORF represents the first example of a short internal ORF functioning as a repressor of a maternal gene in a plant.