We characterized members of the LINE (UnaL2) and SINE (UnaSINE1) families from the eel genome and found that these LINE/SINE partners share similar 3' tails. A retrotransposition assay in HeLa cells demonstrated that the 3' conserved tail of UnaL2 is necessary for its retrotransposition. This 3' tail is recognized in trans by the UnaL2 reverse transcriptase at a surprisingly high rate, and that of UnaSINE1 can also be recognized, thus providing experimental evidence that a SINE can be mobilized by the retrotransposition machinery of a partner LINE. We also demonstrated that short repeats at the 3' end of UnaL2 are required for retrotransposition suggesting that UnaL2 retrotransposes in a manner reminiscent of the reverse transcriptase activity of telomerases.

Long interspersed elements (LINEs) are transposable elements that proliferate within eukaryotic genomes, having a large impact on eukaryotic genome evolution. LINEs mobilize via a process called retrotransposition. Although the role of the LINE-encoded protein(s) in retrotransposition has been extensively investigated, the participation of host-encoded factors in retrotransposition remains unclear. To address this issue, we examined retrotransposition frequencies of two structurally different LINEs—zebrafish ZfL2-2 and human L1—in knockout chicken DT40 cell lines deficient in genes involved in the non-homologous end-joining (NHEJ) repair of DNA and in human HeLa cells treated with a drug that inhibits NHEJ. Deficiencies of NHEJ proteins decreased retrotransposition frequencies of both LINEs in these cells, suggesting that NHEJ is involved in LINE retrotransposition. More precise characterization of ZfL2-2 insertions in DT40 cells permitted us to consider the possibility of dual roles for NHEJ in LINE retrotransposition, namely to ensure efficient integration of LINEs and to restrict their full-length formation.

No abstract

CR1 elements are a family of retroposons. They are classified as long interspersed elements (LINEs) or non-long-terminal-repeat (non-LTR) retrotransposons, and they have been found in the genomes of many vertebrates. However, they have been only partially characterized, and only a 2-kb region of the 3' end of chicken CR1 has been sequenced. In the present study, we determined the entire consensus sequence of CR1 elements in the turtle genome, designated PsCR1. The first open reading frame (ORF1) of PsCR1 has two unusual arrangements of Cys residues. One of them includes a zinc finger motif, CX2CX14CX2C. The putative zinc finger has cysteine residues with identical spacing and a similar amino acid composition to those found in the species-specific transcription initiation factors SL1 and TIF-IB. The 5' untranslated region (5' UTR) of PsCR1 contains a sequence similar to part of the human L1 promoter, L1 site A, and several cis elements of the type found in eukaryotic genes. Within a region of about 500 bp, there are nine "E boxes," cis elements that are recognized by the basic helix-loop-helix (bHLH) family of proteins. This observation raises the possibility that cellular transcription factors that bind to these sequences might act in concert to regulate the expression of PsCR1. The extent of the sequence divergence of the 3' UTR of CR1 between species was found to be lower than the rate of nonsynonymous substitutions per site in ORF2, suggesting that a strict functional constraint must exist for this region. This result strongly suggests that the conserved 3'-end sequence of CR1 is the recognition site for the reverse transcriptase of CR1. A discussion is presented of a possible mechanism for the integration of CR1 elements and also of the intriguing possible recruitment of the reverse transcriptase for the retroposition of SINEs.

The biosynthesis of female moth sex pheromone blends is controlled by a number of different enzymes, many of which are encoded by members of multigene families. One such multigene family, the acyl-CoA desaturases, is composed of certain genes that function as key players in moth sex pheromone biosynthesis. Although much is known regarding the function of some of these genes, very little is known regarding how novel genes have evolved within this family and how this might impact the establishment of new sex pheromone blends within a species. We have discovered that several cryptic ⌬11 and ⌬14 desaturase genes exist in the genomes of the European and Asian corn borers (Ostrinia nubilalis and Ostrinia furnacalis, respectively). Furthermore, an entirely novel class of desaturase gene has arisen in the Ostrinia lineage and is derived from duplication of the ⌬11 desaturase gene and subsequent fusion with a retroposon. Interestingly, the genes have been maintained over relatively long evolutionary time periods in corn borer genomes, and they have not been recognizably pseudogenized, suggesting that they maintain functional integrity. The existence of cryptic desaturase genes in moth genomes indicates that the evolution of moth sex pheromone desaturases in general is much more complex than previously recognized.