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Zusammenfassung:
Plant microRNAs (miRNAs) are produced from precursors that contain self-complementary foldbacks. These precursors are processed by DICER-LIKE1 (DCL1), generating the mature miRNAs that are incorporated into the RISC, a protein complex that regulates miRNA target genes. In plants, some young miRNA genes arose by inverted duplication of what later might become target genes of the miRNAs. However, analysis of the evolutionary origin of young miRNAs suggests that this is not the only path to the emergence of new miRNAs. In silico folding of a typical plant genome results in hundreds of thousands of potential foldback sequences. To test if such random foldbacks could give origin to new miRNAs genes, we previously used data from massively parallel signature sequencing (MPSS) to identify new potential Arabidopsis thaliana miRNA genes that are not conserved in other genomes. These potential miRNA precursors were assayed for both their ability to generate a miRNA and potential effects on target mRNAs. Analysis of these potential miRNAs together with database entries of miRNAs that are private to A. thaliana (i.e., not found in poplar or rice) suggest that while some arose from a sequence that either has self-complementarity by chance or that represents a highly degenerate inverted duplication. We propose that miRNAs can arise spontaneously from foldback sequences captured by transcriptional regulatory sequences. Subsequent stabilization through co-evolution with potential targets may lead to subsequent fixation of a small subset.
