Mutator-like transposable elements (MULEs) are widespread in plants and the majority have long terminal inverted repeats (TIRs), which distinguish them from other DNA transposons. It is known that the long TIRs of Mutator elements harbor transposase binding sites and promoters for transcription, indicating that the TIR sequence is critical for transposition and for expression of sequences between the TIRs. Here, we report the presence of MULEs with multiple TIRs mostly located in tandem. These elements are detected in the genomes of maize, tomato, rice, and Arabidopsis. Some of these elements are present in multiple copies, suggesting their mobility. For those elements that have amplified, sequence conservation was observed for both of the tandem TIRs. For one MULE family carrying a gene fragment, the elements with tandem TIRs are more prevalent than their counterparts with a single TIR. The successful amplification of this particular MULE demonstrates that MULEs with tandem TIRs are functional in both transposition and duplication of gene sequences. 1. Introduction Transposable elements (TEs) are DNA fragments that are capable of moving from one genomic location to another and increasing their copy numbers. Based on their transposition mechanisms, TEs fall into two classes: (1) Class I elements, or retrotransposons, that use the element-encoded mRNA as the transposition intermediate and (2) Class II elements, or DNA transposons, that transpose through a DNA intermediate. Autonomous transposons encode transposases that are responsible for the transposition of themselves and their cognate nonautonomous elements that do not encode transposases. A common feature for DNA transposons, with a few exceptions, is the presence of a terminal inverted repeat (TIR) at each terminus of the element. As an essential structural component of the element, TIR plays important roles in transposition. For example, the transposase encoded by the bacterial Tn3 element specifically binds to its TIR (38？bp in length), and this then facilitates the nicking at the end of Tn3 by DNase I and initializes the transposition process [1]. In eukaryotes, it was shown that the transposase of the Hermes element binds to its imperfect TIRs and excises the element. This process is accompanied by the formation of a hairpin structure in the flanking donor sequence, resembling the V(D)J recombination process [2]. Binding of transposase to the TIR and to the target DNA mediates the synapsis of the transposon ends and the target DNA, allowing the insertion of the element into the target sequence
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