| Summary: | The <I>mariner </I> transposable element <I>mos</I>1, isolated from <I>Drosophila mauritiana</I> belongs to a family of inverted repeat DNA transposons, found in many phyla. Members of this family of elements transpose by a 'cut and paste' mechanism, in which the element is excised from its position in the genome and inserted elsewhere. <I>Mos</I>1 is 1286bp long, and contains a single open reading frame of 345 amino acids. This ORF encodes a transposase which is essential for the transposition mechanism. The existence of various forms of extrachromosomal copies of transposons has been documented for a number of elements and it has been assumed that some of these are intermediates in transposition. Active <I>mariner</I> elements exist in the genome of <I>D. simulans</I>. Several forms of free copies of <I>mariner</I> have been isolated from DNA prepared from this species. Some of these are circular in conformation. Sequence analysis, performed to determine the structures across junctions of the circular copies, has revealed that all elements are deleted for some <I>mariner</I> nucleotides. The majority of elements are missing three bases from one or the other end. Recombinant <I>mos</I>1 transposase purified from <I>E. coli</I> is able to stimulate excision of <I>mos</I>1 elements from plasmids. These excised copies are linear in structure. Primer extension analysis has been employed to investigate the terminal most nucleotides of these elements. The excision of <I>mos</I>1 occurs, leaving the terminal most three base pairs from each 5' strand behind in the plasmid. No circular copies were detected in this reaction. An explanation for these findings is discussed and a model for transposition of <I>mos</I>1 and <I>mariner </I>elements from <I>D. simulans</I> is proposed.
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