Mobility of mPing and its associated elements is regulated by both internal and terminal sequences

DNA transposable elements are mobilized by a "cut and paste" mechanism catalyzed by the binding of one or more transposase proteins to terminal inverted repeats (TIRs) to form a transpositional complex. Study of the rice genome indicates that the mPing element has experienced a recent burst in transposition compared to the closely related Ping and Pong elements. A previously developed yeast transposition assay allowed us to probe the role of both internal and terminal sequences in the mobilization of these elements.

We observed that mPing and a synthetic mPong element have significantly higher transposition efficiency than the related autonomous Ping and Pong elements. Systematic mutation of the internal sequences of both mPing and mPong identified multiple regions that promote or inhibit transposition. Simultaneous alteration of single bases on both mPing TIRs resulted in a significant reduction in transposition frequency, indicating that each base plays a role in efficient transposase binding. Testing chimeric mPing and mPong elements verified the important role of both the TIRs and internal regulatory regions. Previous experiments showed that the G at position 16, adjacent to the 5' TIR, allows mPing to have higher mobility. Alteration of the 16th and 17th base from mPing's 3' end or replacement of the 3' end with Pong 3' sequences significantly increased transposition frequency.

As the transposase proteins were consistent throughout this study, we conclude that the observed transposition differences are due to the element sequences. The presence of sub-optimal internal regions and TIR bases supports a model in which transposable elements self-limit their activity to prevent host damage and detection by host regulatory mechanisms. Knowing the role of the TIRs, adjacent sub-TIRs, and internal regulatory sequences allows for the creation of hyperactive elements.