Transcriptional analysis of the HeT-A retrotransposon in mutant and wild type stocks reveals high sequence variability at Drosophila telomeres and other unusual features
14
Citation
31
Reference
10
Related Paper
Citation Trend
Abstract:
Abstract Background Telomere replication in Drosophila depends on the transposition of a domesticated retroelement, the HeT-A retrotransposon. The sequence of the HeT-A retrotransposon changes rapidly resulting in differentiated subfamilies. This pattern of sequence change contrasts with the essential function with which the HeT-A is entrusted and brings about questions concerning the extent of sequence variability, the telomere contribution of different subfamilies, and whether wild type and mutant Drosophila stocks show different HeT-A scenarios. Results A detailed study on the variability of HeT-A reveals that both the level of variability and the number of subfamilies are higher than previously reported. Comparisons between GIII, a strain with longer telomeres, and its parental strain Oregon-R indicate that both strains have the same set of HeT-A subfamilies. Finally, the presence of a highly conserved splicing pattern only in its antisense transcripts indicates a putative regulatory, functional or structural role for the HeT-A RNA. Interestingly, our results also suggest that most HeT-A copies are actively expressed regardless of which telomere and where in the telomere they are located. Conclusions Our study demonstrates how the HeT-A sequence changes much faster than previously reported resulting in at least nine different subfamilies most of which could actively contribute to telomere extension in Drosophila. Interestingly, the only significant difference observed between Oregon-R and GIII resides in the nature and proportion of the antisense transcripts, suggesting a possible mechanism that would in part explain the longer telomeres of the GIII stock.Keywords:
Retrotransposon
Retroposon
A retrotransposon is a segment of DNA that can replicate itself within genomes through the process of retrotransposition . The reverse transcriptase enzyme is used by retroviruses to integrate a copy of their RNA sequence into a host genome. Seventy percent of the human genome is derived from transposable elements, compared with just 2 percent which actually codes for a protein. LINE1 elements and Alu elements are two of the most common types of retrotransposons in the human genome and novel LINE‐1 and Alu insertions continue to accumulate. Active retrotransposition provides one of the mechanisms by which populations acquire novel genetic variation, novel patterns of gene regulation, and novel phenotypes.
Retrotransposon
Retroposon
Alu element
Interspersed repeat
Cite
Citations (0)
Retrotransposons are transposable elements that are transposed via transcription and reverse transcription. Their copies have accumulated in the genome of mammals, occupying approximately 40% of mammalian genomic mass. These copies are often involved in numerous phenomena, such as chromatin spatial organization, gene expression, development and disease, and have been recognized as a driving force in evolution. Different organisms have gained specific retrotransposon subfamilies and retrotransposed copies, such as hundreds of Mus-specific subfamilies with diverse sequences and genomic locations. Despite this complexity, basic information is still necessary for present-day genomic and epigenomic studies. Herein, we describe the characteristics of each subfamily of Mus-specific retrotransposons in terms of sequence structure, phylogenetic relationships, evolutionary age, and preference for A or B compartments of chromatin.
Retrotransposon
Retroposon
Subfamily
Genomic Organization
Epigenomics
Cite
Citations (3)
Retrotransposon
Telomere-binding protein
Cite
Citations (39)
During the course of a random sequencing project of the genome of the dimorphic yeast Yarrowia lipolytica, we have identified sequences that were repeated in the genome and that matched the reverse transcriptase (RT) sequence of non–long terminal repeat (non–LTR) retrotransposons. Extension of sequencing on each side of this zone of homology allowed the definition of an element over 6 kb long. The conceptual translation of this sequence revealed two open reading frames (ORFs) that displayed several characteristics of non–LTR retrotransposons: a Cys-rich motif in the ORF1, an N-terminal endonuclease, a central RT, and a C-terminal zinc finger domain in the ORF2. We called this element Ylli (for Y. lipolyticaLINE). A total of 19 distinct repeats carrying the 3′ untranslated region (UTR) and all ending with a poly-A tail were detected. Most of them were very short, 17 being 134 bp long or less. The number of copies of Ylli was estimated to be around 100 if these short repeats are 5′ truncations. No 5′ UTR was clearly identified, indicating that entire and therefore active elements might be very rare in the Y. lipolytica strain tested. Ylli does not seem to have any insertion specificity. Phylogenetic analysis of the RT domain unambiguously placed Ylli within the L1 clade. It forms a monophyletic group with the Zorro non–LTR retrotransposons discovered in another dimorphic yeast Candida albicans. BLAST comparisons showed that ORF2 of Ylli is closely related to that of the slime mold Dictyostelium discoideum L1 family, TRE.
Retrotransposon
Yarrowia
ORFS
Retroposon
Interspersed repeat
Cite
Citations (34)
Retrotransposon
Retroposon
Cite
Citations (2)
The term “C-value paradox” was coined by C. A. Thomas, Jr. in 1971 [Thomas CA (1971) Ann Rev Genetics 5:237–256] to describe the initially puzzling lack of correlation between an organism's genome size and its morphological complexity. Polyploidy and the expansion of repetitive DNA, primarily transposable elements, are two mechanisms that have since been found to account for this differential. While the inactivation of retrotransposons by methylation and their removal from the genome by illegitimate recombination have been well documented, the cause of the apparently periodic bursts of retrotranposon expansion is as yet unknown. We show that the expansion of the CRM1 retrotransposon subfamily in the ancient allotetraploid crop plant corn is linked to the repeated formation of novel recombinant elements derived from two parental retrotransposon genotypes, which may have been brought together during the hybridization of two sympatric species that make up the present day corn genome, thus revealing a unique mechanism linking polyploidy and retrotransposition.
Retrotransposon
Retroposon
Interspersed repeat
Subfamily
Cite
Citations (47)
Insertion of the human non-LTR retrotransposon LINE-1 (L1) into chromosomal DNA is thought to be initiated by a mechanism called target-primed reverse transcription (TPRT). This mechanism readily accounts for the attachment of the 3'-end of an L1 copy to the genomic target, but the subsequent integration steps leading to the attachment of the 5'-end to the chromosomal DNA are still cause for speculation. By applying bioinformatics to analyze the 5' junctions of recent L1 insertions in the human genome, we provide evidence that L1 uses at least two distinct mechanisms to link the 5'-end of the nascent L1 copy to its genomic target. While 5'-truncated L1 elements show a statistically significant preference for short patches of overlapping nucleotides between their target site and the point of truncation, full-length insertions display no distinct bias for such microhomologies at their 5'-ends. In a second genome-wide approach, we analyzed Alu elements to examine whether these nonautonomous retrotransposons, which are thought to be mobilized through L1 proteins, show similar characteristics. We found that Alu elements appear to be predominantly integrated via a pathway not involving overlapping nucleotides. The results indicate that a cellular nonhomologous DNA end-joining pathway may resolve intermediates from incomplete L1 retrotransposition events and thus lead to 5' truncations.
Retrotransposon
Alu element
Retroposon
Cite
Citations (108)
A new long terminal repeat (LTR) retrotransposon, named REM1, has been identified in the green alga Chlamydomonas reinhardtii. It was found in low copy number, highly methylated, and with an inducible transpositional activity. This retrotransposon is phylogenetically related to Ty3-gypsy LTR retrotransposons and possesses new and unusual structural features. A regulatory module, ORF3p, is present in an inverse transcriptional orientation to that of the polyprotein and contains PHD-finger and chromodomains, which might confer specificity of the target site and are highly conserved in proteins involved in transcriptional regulation by chromatin remodeling. By using different wild-type and mutant strains, we show that CrREM1 was active with a strong transcriptional activity and amplified its copy number in strains that underwent foreign DNA integration and/or genetic crosses. However, integration of CrREM1 was restricted to these events even though the expression of its full-length transcripts remained highly activated. A regulatory mechanism of CrREM1 retrotransposition which would help to minimize its deleterious effects in the host genome is proposed.
Retrotransposon
Chlamydomonas reinhardtii
Retroposon
Cite
Citations (28)
Eukaryotic and prokaryotic genomes encode either Type I or Type II Ribonuclease H (RNH) which is important for processing RNA primers that prime DNA replication in almost all organisms. This review highlights the important role that Type I RNH plays in the life cycle of many retroelements, and its utility in tracing early events in retroelement evolution. Many retroelements utilize host genome-encoded RNH, but several lineages of retroelements, including some non-LTR retroposons and all LTR retrotransposons, encode their own RNH domains. Examination of these RNH domains suggests that all LTR retrotransposons acquired an enzymatically weak RNH domain that is missing an important catalytic residue found in all other RNH enzymes. We propose that this reduced activity is essential to ensure correct processing of the polypurine tract (PPT), which is an important step in the life cycle of these retrotransposons. Vertebrate retroviruses appear to have reacquired their RNH domains, which are catalytically more active, but their ancestral RNH domains (found in other LTR retrotransposons) have degenerated to give rise to the tether domains unique to vertebrate retroviruses. The tether domain may serve to control the more active RNH domain of vertebrate retroviruses. Phylogenetic analysis of the RNH domains is also useful to “date” the relative ages of LTR and non-LTR retroelements. It appears that all LTR retrotransposons are as old as, or younger than, the “youngest” lineages of non-LTR retroelements, suggesting that LTR retrotransposons arose late in eukaryotes.
Retrotransposon
Retroposon
Endogenous retrovirus
RNase H
Cite
Citations (36)
Penelope-like elements (PLEs) are a relatively little studied class of eukaryotic retroelements, distinguished by the presence of the GIY-YIG endonuclease domain, the ability of some representatives to retain introns, and the similarity of PLE-encoded reverse transcriptases to telomerases. Although these retrotransposons are abundant in many animal genomes, the reverse transcriptase moiety can also be found in several protists, fungi, and plants, indicating its ancient origin. A comprehensive phylogenetic analysis of PLEs was conducted, based on extended sequence alignments and a considerably expanded data set. PLEs exhibit the pattern of evolution similar to that of non-LTR retrotransposons, which form deep-branching clades dating back to the Precambrian era. However, PLEs seem to have experienced a much higher degree of lineage losses than non-LTR retrotransposons. It is suggested that PLEs and non-LTR retrotransposons are included into a larger eTPRT (eukaryotic target-primed) group of retroelements, characterized by 5′ truncation, variable target-site duplication, and the potential of the 3′ end to participate in formation of non-autonomous derivatives.
Retrotransposon
Retroposon
Lineage (genetic)
Mega-
Cite
Citations (70)