Principal Investigator Mary-Lou Pardue
cause these elements transpose onto chromosome ends, their 5’ DNA is exposed to terminal erosion until another element transposes onto the chromosome and becomes the new end. As a result, telomere arrays contain elements with varying amounts of 5’ truncation. Unlike telomerase-maintained telomeres, these arrays must also provide a breeding stock of elements capable of transposing to extend chromosome ends. Therefore, terminal telomeric elements must occasionally escape loss of essential 5’ sequence, and we found that they have evolved novel ways to do just that.
The promoter described above for D. melanogaster HeT-A and D. virilis TART does this by starting transcription in the 3’UTR of the upstream element. The short segment of 3’UTR from the upstream element added to the transcript serves to buffer end erosion when that transcript is reverse-transcribed onto the chromosome. In telomere arrays complete copies of these elements retain variable amounts of this expendable 3’ sequence at their 5’ends, showing that this mechanism has indeed provided protection.
D. melanogaster TART also needs 5’end protection. Somewhat surprisingly, it does not use the mechanism used both by its D. virilis homologue and by its partner in the D. melanogaster telomere. However TARTmel has several unusual features which allowed us to propose a model for its 5’ protection. All presumably complete TARTs sequenced have different length 5’UTRs. These range from 33 bp to 3934 bp with no apparent length clustering. Furthermore, each 5’UTR is completely identical to sequence in the 3’UTR of its own element, strong evidence that the two sequences are evolving together. TARTmel has only one transcription start site for its transposition-intermediate RNA. This start site is ~75 bp upstream from the start of the coding region, thus the longest 5’UTR of each element should contain ~75 bp, yet all but one of the sequenced elements have much longer 5’UTRs. We proposed that the unexpected extra 5’ sequence is added when TARTmel is reverse transcribed onto the end of the chromosome. In this model the reverse transcriptase makes a template jump from the 5’ end of the RNA back to the same sequence in the 3’UTR and recopies that to add redundant sequence to the 5’ end of the element.