Site-specific non-LTR retrotransposons

DNA transposons are the mobile elements that move by a "cut and paste" mechanism (1 , 2). In contrast, retrotransposons encode reverse transcriptase, and move by a "copy and paste" mechanism. The process of retrotransposon insertion into genomic locations involves an RNA intermediate. Retrotransposons can be classified into long terminal repeat (LTR) and non-LTR retrotransposons. LTR retrotransposons have LTRs at both ends and resemble retroviruses in both structure and integration mechanisms. Non-LTR retrotransposons comprise two subtypes, long interspersed nuclear elements (LINEs) and short interspersed nuclear elements (SINEs). Non-LTR retrotransposons are in general 4 to 7 kb long and do not carry LTRs, and their retrotransposition mechanism is different from that of LTR retrotransposons. SINEs are nonautonomous retrotransposons of 100 to 500 bp that do not encode proteins. It has been proposed that the proteins encoded by LINEs are the source of the enzymatic retrotransposition machinery of SINEs (3 , 4 , 5).