Drosophila telomeres: an exception providing new insights

Abstract

Drosophila telomeres comprise DNA sequences that differ dramatically from those of other eukaryotes. Telomere functions, however, are similar to those found in telomerase-based telomeres, even though the underlying mechanisms may differ. Drosophila telomeres use arrays of retrotransposons to maintain chromosome length, while nearly all other eukaryotes rely on telomerase-generated short repeats. Regardless of the DNA sequence, several end-binding proteins are evolutionarily conserved. Away from the end, the Drosophila telomeric and subtelomeric DNA sequences are complexed with unique combinations of proteins that also modulate chromatin structure elsewhere in the genome. Maintaining and regulating the transcriptional activity of the telomeric retrotransposons in Drosophila requires specific chromatin structures and, while telomeric silencing spreads from the terminal repeats in yeast, the source of telomeric silencing in Drosophila is the subterminal arrays. However, the subterminal arrays in both species may be involved in telomere-telomere associations and/or communication.

Figure 1

Schematic comparison of the overall structure of the telomere region in Drosophila and in human and their associated proteins. In Drosophila, the terminal capping complex (blue square) consists of proteins that are primarily associated with heterochromatin and bind in a sequence-independent manner to the end of the retrotransposon (HTT) array, which replaces the telomerase-generated repeats found in other eukaryotes. This array consists of tandem non-LTR retrotransposons of irregular length with their oligo(A) tails (indicated by A) facing towards the centromere. It is associated with proteins that are otherwise found in either heterochromatin or euchromatin (green rectangle) and has euchromatic properties. The subtelomeric (TAS) repeats (alternating light and dark blue small rectangles) assemble a distinct silencing complex (orange oval), which spreads into the proximal region of the HTT array. Thus, the capping functions at the chromosome end are clearly separated from the silencing domain of TAS, which has heterochromatic properties. In human cells, the telomerase-generated TTAGGG repeats (alternating black and grey bar) may form a t-loop structure, which is stabilized by specific proteins (red oval). Proteins binding to the telomeric repeats assemble a heterochromatic silencing complex (telosome or shelterin; purple ovals), which may spread into the TAS region. Thus, capping and silencing are centered on the telomeric repeats. Associated proteins are shown below each structure. Not shown: the cap and shelterin are maintained by DNA damage response proteins, ATM and the MRN complex.

Figure 2

Schematic of the nuclear and cytoplasmic events in Drosophila telomere elongation by retrotransposition. Transcripts are generated from a telomeric retrotransposon using promoter activity located in the 3′ UTR of an upstream HeT-A element (black arrows). The transcripts leave the nucleus to serve as mRNA for the translation of the element-encoded GAG-like protein (red squiggle), which contains three nuclear localization signals. GAG binds the RNA and facilitates re-entry into the nucleus. After docking to a chromosome end, perhaps mediated by a protein-protein interaction between the GAG-protein and the terminal-capping complex, a reverse transcriptase uses the free 3′ hydroxyl group at the chromosome end as primer to copy the RNA intermediate into the first DNA strand. Second strand synthesis occurs by DNA repair and completes the addition of a new HeT-A retrotransposon. It is conceivable that HTT transcripts that have not left the nucleus may also be used a templates for reverse transcription at the chromosome ends. However, sequence analyses of very recently transposed HeT-A elements (15) and of several in native telomeric arrays (58) suggest that there is a selection for the incorporation of elements with a functional GAG open reading frame that have previously been translated in the cytoplasm. Such a positive selection would prevent a gradual inactivation of telomeric transposons due to the accumulation of mutations. Other symbols are as in Fig. 1.

Figure 3

Cis-acting silencing at the telomeres of Drosophila as deduced from expression levels of P element insertions carrying a white (w) reporter gene in various telomeric domains. Silencing effects (orange oval) emerge from TAS and spread a short distance into the HTT array. Two different insertion constructs were used. The EPgy2 element lacks the eye-specific enhancer and expresses w at a relatively low level; the SuPor-P element carries the eye-specific enhancer and expresses w more strongly. Insertions in TAS and in HTT close to TAS are repressed, while those in HTT far from TAS are not. Other symbols are as in Fig. 1.

Figure 4

Trans-effect of TAS deletions on the expression of telomeric transgenes. w transgene expression at the 2L telomere occurs primarily from the w promoter (orange bent arrow) with a small contribution from the adjacent HeT-A promoter (blue bent arrow). This expression is repressed by the adjacent TAS in cis (orange oval) in the presence of a full length TAS array (red line). Deletion of 2L TAS in trans (Df(2L)M26) causes higher expression (green round arrow) of the w transgene. Others symbols as in Fig. 1