Transposition mechanism of DNA-type transposable elements of the medaka fish

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For the purpose of studying the transposition mechanisms of the medaka fish transposable element Tol2, we first established an experimental system to obtain the excision rate of the element, in which the bacterial lacZ gene on a plasmid vector was used as an indicator of excision. In this system, loss of the Tol2 element due to its excision results in blue bacterial colonies. Next, using this system, we identified an autonomous Tol2 copy, Tol2 is present in the genome of the medaka fish (O.latipes) but is not in the Luzon medaka fish (O.luzonensis) which is closely related to the medaka fish. Therefore, excision of a single element introduced into the Luzon medaka fish indicates that the element is autonomous. It was successful to detect excision, and, thus, the copy used was found to be an autonomous element.After these preparations, we started examining portions of the autonomous Tol2 copy for their importance on the transposition function. We made various clones that carry sequence alterations or deletions, and estimated the transposition rate of the clones. The following results were obtained : (1) The terminal inverted repeat of the Tol2 element are required for transposition. (2) A factor necessary for transposition is present in the central region of the element. (3) The length of the element is positively correlated with the transposition rate. The last two things are unique to Tol2, that is, other transposable elements so far examined do not have these characters.We also examined variation in the structure of Tol2 elements in a medaka fish natural population. Tol2 was found to be highly homogeneous in length and the structure. This is consistent with the results about the transposition mechanisms described above.