Fig. 1.

Major transposon types. Among the Class I elements, non-LTR retrotransposons encode two proteins required for retrotransposition: ORF1, a non-specific nucleic acid binding protein, and ORF2 which has endonuclease (EN) and reverse transcriptase (RT) activities. In their simplest form, LTR retrotransposons have a gag gene that encodes structural proteins, and a pol gene that encodes a protease (PR), integrase (IN), and an RT, bounded by LTRs. Class II elements use only DNA intermediates, and although eukaryotic DNA transposons usually encode only the transposase enzyme required for movement (depicted in light blue, and shown in the inset for cut-and-paste DNA transposons), prokaryotic DNA transposons often carry other genes such as those encoding antibiotic resistance proteins (shown in varying shades). Specific binding sites for the transposase (in orange) are located at each end of the transposon.