Fig. 4.

Proposed mechanism for extending the 5′ end of D. melanogaster TART. Transcription starts (bent arrow) near the ATG of ORF 1 (gag), producing a transposition intermediate RNA (dashed black line) lacking most of the 5′ UTR. This RNA has a small piece of the parent element’s 5′ PNTR (short gold arrow) and a complete 3′ PNTR (long gold arrow). Steps 1–3 show the RNA as it is reverse-transcribed into DNA on the chromosome end. (Step 1) The polyA tail associates with the chromosomal DNA (magenta), and RT begins to copy the RNA. The gray oval represents proteins proposed to hold the RNA in a conformation that brings the 5′ PNTR sequence into proximity to the 3′ end of the 3′ PNTR (omitted for clarity in later steps). (Step 2) When RT reaches the 5′ end of the transcript, it makes a template jump back to the matching 3′ end of the 3′ PNTR. (Step 3) RT dissociates the RNA–DNA complex and recopies some or all of the 3′ PNTR. As a result, the transposed element will have more 5′ UTR sequence than the RNA did and possibly more sequence and longer PNTRs than the element from which it was derived.