Genomic Distribution and Organization of MITE Sequences in Graminae Species

摘要

Although the presence of transposable elements (TEs) in the genome has been known more than fifty years, their biological functions have not been clear. However, we know that the major proportion of the flowering plant genomes are composed of numerous of families of TEs, which are the main cause of the wide C-value differences in plant species. There are two broad groups of TEs, each with different characteristics. The Class 1 or retroelements are the TEs which can mobilize themselves as RNA intermediates. In this group, two kinds of retroelements were known, namely LINEs (long interspersed nuclear elements) and SINEs (short interspersed elements). Lake the Alu families, these elements are rather abundant in mammalian genomes compared to the plant genomes. Class 2 or DNA elements mobilize themselves via DNA intermediates. Class 2 elements are characterized by terminal invert repeats. Class 2 can also be categorized into two groups based on the transposability. If the element can transpose itself, it is an autonomous element like Ac and Spm in maize. If it needs another element to transpose, it is a non-autonomous element like Ds and dSpm in maize. Recently, new families of transposable elements having invert repeats in their terminal ends were found in many plant species. Because the size of these new families are rather small, this is called as MITE (miniature transposable elements). MITEs have several common features such as that they are short (less than 500bp), having no coding potential, conserved terminal invert repeat (TIR) and target site preference by having two or three bases of direct repeat (DIR) at their both ends. Although MITEs were known to be widely present in monocots and dicots, more families have been identified in Graminae species. Five different MITE families (Gaijin, Castaway, Ditto, Wanderer, and Explorer) were found in rice genome. In addition to these, Tourist, Stoaway and Heartbreaker families are known to be present in cereal species . Although the cumulative evidence is not enough to draw conclusion about the role of MITEs in genome evolution, its involvement in gene regulation was put forwatd since MITE se-quences were found to be present in the introns or near the coding regions of genes. However, the prevalence in various of cereal genomes can make wild speculations on the transposition in the genomes and the roles for adaptation during evolution. The Ditto-Os2 appeared to have comparatively high homology to the maize homeobox gene Knotted-1. The Gaijin-So1, sequence from sorghum, was present almost in entire region of the 3' -untranslated region of glucose transporter genes, which strongly implied that the Gaijin-Sol might be involved in this gene's polyadenylation signal and site. Most of the plant transposable elements are present more than 100 copies per geneome. However, MITEs that are present in high copy numbers (1 000 ～ 15 000 copies per haploid genome) should be the major repetitive DNA families in plants.