Another miRNA family, JAW/miR159, which negatively regulates several members of the TCP and MYB transcription factor families is involved in leaf development ( Palatnik et al., 2003). A family of miRNA, miR172 negatively regulates APETALA2-like transcription factors, thus controlling flowering time and floral organ identity ( Aukerman and Sakai, 2003 Chen,2004). For example, the shoot apical meristem and a related structure, the floral meristem, initiate primordia of lateral organs such as leaves, sepals or stamens. Organogenesis in plants, in contrast to animals, proceeds throughout their life span as new tissues and organs are continuously produced by meristems. In plants, most of the miRNA targets predicted by bioinformatics are transcription factors involved in the control of development, raising the possibility that miRNAs may play an important role in this process( Rhoades et al., 2002). Conversely, plant miRNAs are perfectly or almost perfectly complementary to their targets ( Rhoades et al., 2002), and their interaction triggers the cleavage of the mRNA ( Han et al., 2004 Kasschau et al., 2003 Llave et al., 2002 Palatnik et al., 2003 Tang et al., 2003 Vazquez et al., 2004 Xie et al., 2003), although examples of translational attenuation have also been reported( Aukerman and Sakai, 2003 Chen, 2004). Interaction of the miRNA with imperfect complementary sequences located in the 3′ untranslated region (UTR) of the target mRNAs leads to translational attenuation. Although the cleavage affects both strands of the hairpin precursors, only one strand, the mature miRNA, is preferentially accumulated and incorporated into a ribonucleoprotein complex, the miRNP complex( Khvorova et al., 2003 Mourelatos et al., 2002 Schwarz et al., 2003). Animal miRNAs are transcribed as long primary transcripts (pri-miRNAs) that are first processed into hairpin precursors of about 70 nucleotides (pre-miRNAs) and then into mature miRNAs. MiRNAs are small, single-stranded RNAs of about 21 nucleotides found in both animals and plants that post-transcriptionally regulate gene expression(for reviews, see Bartel, 2004 Lai, 2003).
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