To understand the complex functions required for the propagation of retroelements we study retrotransposons, a family of elements that are closely related to retroviruses. A significant advantage of studying retrotransposons is that they exist in hosts such as yeast that can readily be studied using sophisticated molecular genetic techniques. In the process of characterizing transposition in Schizosaccharomyces pombe, we have obtained strong evidence that the reverse transcriptase of the retrotransposon Tf1 uses a novel mechanism of self-priming to initiate cDNA synthesis. This is in contrast to the tRNA mechanisms thought to be used by all other LTR-containing elements. The first 11 bases of the Tf1 transcript anneal to the primer binding site and a cleavage reaction between the 11th and 12th nucleotides results in the initiation of reverse transcription. This region of the mRNA forms a complex RNA structure composed of 27 basepairs that includes the primer binding site and the first 11 nucleotides. We have shown previously that the formation of the 27 basepair structure is required for reverse transcription. We have identified an additional structure composed of a 7 basepair stem and a 25 nucleotide loop. Extensive mutagenesis of this stem-loop indicated that it too is required for reverse transcription in vivo. The position, size and function of this stem-loop in the mRNA of Tf1 is analogous to the U5-IR stem-loop of Rous sarcoma virus. The U5-IR stem-loop has also been identified in several other retroviruses suggesting that this structure has been highly conserved. In an effort to identify host factors that contribute to the transposition of Tf1, we have isolated mutated strains of S. pombe that are defective for transposition. Mutations in three genes, hop1, hop2, and hop3, greatly reduce the frequency of Tf1 transposition. As the result of large scale complementation screens, two of these genes have now been cloned. The sequence of hop3 indicated that it is a homologue of the sin3 genes found in S. cerevisiae and mammals. Because sin3 proteins are known to contribute to the deacetylation of histones and the condensation of chromatin, we are studying the possibility that Tf1 integration requires specific types of chromatin structure. The sequence of hop5 indicated that this gene is closely related to a class of nucleoporins that contain multiple copies of FXFG repeats. We are pursuing the model that hop5 is a nuclear pore factor that is required for the nuclear import of Tf1 integrase and reverse transcripts.