Terminal-repeat retrotransposons in miniature (TRIM) are involved in restructuring plant genomes

A new group of long terminal repeats (LTR) retrotransposons, termed termina l-repeat retrotransposons in miniature (TRIM), are described that are prese nt in both monocotyledonous and dicotyledonous plant. TRIM elements have te rminal direct repeat sequences between approximate to 100 and 250 bp in len gth that encompass an internal domain of approximate to 100-300 bp. The int ernal domain contains primer binding site and polypurine tract motifs but l acks the coding domains required for mobility. Thus TRIM elements are not c apable of autonomous transposition and probably require the help of mobilit y-related proteins encoded by other retrotransposons. The structural organi zation of TRIM elements suggests an evolutionary relationship to either LTR retrotransposons or retroviruses. The past mobility of TRIM elements is in dicated by the presence of flanking 5-bp direct repeats found typically at LTR retrotransposon insertion sites, the high degree of sequence conservati on between elements from different genomic locations, and the identificatio n of related to empty sites (RESites). TRIM elements seem to be involved ac tively in the restructuring of plant genomes, affecting the promoter, codin g region and intron-exon structure of genes. In solanaceous species and mai ze, TRIM elements provided target sites for further retrotransposon inserti ons. In Arabidopsis, evidence is provided that the TRIM element also can be involved in the transduction of host genes.