Trans-mobilization of non-L1 mRNAs by the L1 retrotransposition machinery. The L1-encoded proteins preferentially bind to the RNA that encoded them in cis to promote their retrotransposition by target-site primed reverse transcription (left). Cellular RNAs (e.g., ORF1mneoI, middle; or Alu RNA, right) can occasionally use the L1 RT to mediate their mobility in trans. During target-site primed reverse transcription a template switch from L1 mRNA to U6 snRNA can lead to the formation U6/L1 pseudogenes. The data indicate that U6/ORF1mneoI pseudogenes can be generated by sequential template choice and template-switching mechanisms. However, U6/Alu pseudogenes are not efficiently generated in our experimental system, leading us to propose that ORF1p promotes efficient U6 snRNA retrotransposition. ORF1p molecules are depicted as small ovals associated in trimers (Martin et al. 2003). ORF2p is drawn as a large bolded oval. L1, ORF1mneoI, and Alu mRNA that are being reverse transcribed by target-site primed reverse transcription are indicated. U6 snRNA is indicated in gray toward the bottom of the figure. Whether U6 snRNA gains access to the L1 retrotransposition machinery in the cytoplasm or nucleus remains unknown. As proposed, the Alu RNP complex is shown without ORF1p (Dewannieux et al. 2003). However, it remains possible that some Alu RNA is associated with ORF1p in vivo.
