FIG. 3.

Rearranged versions of the Tf1-neoAI plasmid with the IN frameshift were selected during the recombination assay. The LTR sequences are represented by triangles, the Gag, PR, RT, and IN coding sequences are indicated by circles, and the neo, URA3, and intron positions are marked by rectangles. The arrow indicates that neo transcription proceeds in the opposite direction from Tf1 transcription. The position of the nmt1 promoter is indicated by nmt. Positions of the restriction sites that were used to reveal the structure of these plasmids are shown. Enzymes used: A, AvrII; P, PstI; Bs, BsrGI; C, ClaI; S, SpeI; B, BamHI. (A) One plasmid isolated from a G418^r strain of S. pombe was identical to the parent except that the intron was absent. (B) Two other plasmids isolated from G418^r strains of S. pombe were identical and contained tandem duplications of the transposon. The downstream copies of the neo genes contained introns, while the upstream copies lacked introns. (C) Another plasmid isolated from a G418^r strain of S. pombe contained a tandem duplication of Tf1. Neither copy of neo in this plasmid contained intron sequence.