The Tf2 retrotransposon, found in the fission yeast Schizosaccharomyces pombe, is nearly identical to its sister element, Tf1, in its reverse transcriptase-RNase H and integrase domains but is very divergent in the gag domain, the protease, the 5 untranslated region, and the U3 domain of the long terminal repeats. It has now been demonstrated that a neo-marked copy of Tf2 overexpressed from a heterologous promoter can mobilize into the S. pombe genome and produce true transposition events. However, the Tf2-neo mobilization frequency is 10-to 20-fold lower than that of Tf1-neo, and 70% of the Tf2-neo events are homologous recombination events generated independently of a functional Tf2 integrase. Thus, the Tf2 element is primarily dependent on homologous recombination with preexisting copies of Tf2 for its propagation. Finally, production of Tf2-neo proteins and cDNA was also analyzed; surprisingly, Tf2 was found to produce its reverse transcriptase as a single species in which it is fused to protease, unlike all other retroviruses and retrotransposons.Transposable elements constitute up to 50% of the eukaryotic genome (51, 53). Though they can act as positive forces in the evolution of an organism, both by providing part of the chromosomal architecture (30) and by providing a source of mutagens (1), they also represent a burden on the host cell genome and a potential threat to host cell viability, should they "jump" into an essential region of the genome or mediate a rearrangement thereof. Different transposable elements have developed different means to balance their survival with that of the host cell by controlling their spread within the genome. Most balancing mechanisms described thus far appear to involve control of either transposon expression or target site selection (9). These mechanisms often rely on host-specific factors, such as transcription factors (10, 27), splicing factors (47), and chromatin organization (58). Characterization of different transposable elements can therefore reveal both new mechanisms for control of element mobility and new hostelement interactions.Long terminal repeat (LTR)-type retrotransposons (hereafter referred to simply as retrotransposons) have been isolated from many different eukaryotic organisms, including fruit flies, maize, and the yeast Saccharomyces cerevisiae (6,23,(48)(49)(50). Retrotransposons resemble retroviruses in both genome structure and replication mechanism (9). Like retroviruses, they possess terminally redundant ends (LTRs), a primer binding site for the initiation of reverse transcription, and a polypurine tract that serves as the primer in second-strand synthesis of the cDNA copy of the element. A single mRNA encodes proteins homologous to the retroviral structural proteins capsid (CA) and, sometimes, nucleocapsid (NC) and to the retroviral enzymes protease (PR), reverse transcriptase-RNase H (RT), and integrase (IN). These proteins coassemble to form retrovirus-like particles in which reverse transcription of an RNA intermediate takes place. The res...
