The synthetic element in question, abbreviated smL1, was engineered from a native mouse L1 called L1spa. The rationale behind its construction rested on a striking nucleotide composition feature: L1 open reading frames are unusually rich in adenosine, with roughly 40 percent of their coding sequence composed of the base A. This adenosine richness was suspected to hamper expression in mammalian cells, possibly because A-rich transcripts adopt structures or trigger pathways that reduce mRNA stability or translation. To test this idea, the original designers recoded the L1 open reading frames so that the adenosine content dropped from about 40 percent to 26 percent, a level closer to the average composition of mammalian genes, while preserving the encoded proteins. The result was dramatic. The synthetic element expressed far more mRNA and retrotransposed with markedly higher efficiency than its native parent, a finding that seemed to confirm that adenosine-rich coding sequences constrain L1 activity.

That conclusion, however, rested on the assumption that codon optimization was the only meaningful difference between the two elements. The new work challenges that assumption by taking a closer look at the actual sequence changes introduced during synthesis. Chan and colleagues identified two nonsynonymous substitutions in the ORF1 sequence, meaning that the synthetic element does not encode exactly the same ORF1 protein as L1spa. ORF1p is the RNA-binding protein that coats L1 transcripts and is essential for forming the ribonucleoprotein particle that carries out retrotransposition, so even conservative amino acid changes in this protein could plausibly alter function. More intriguingly, the team noticed that the smL1 construct lacks a conserved guanosine-rich region, or GRR, in its 3 prime untranslated region. This stretch of G-rich sequence has the potential to fold into a G-quadruplex, a four-stranded nucleic acid structure that can influence how transcripts are processed, translated or degraded.

To disentangle these possibilities, the researchers systematically rebuilt the differences between L1spa and smL1, testing each change in isolation and in combination using engineered retrotransposition reporter assays. The outcome was unambiguous: the boost in retrotransposition efficiency seen with smL1 was not attributable to the synthetic, de-adenosinated open reading frames themselves. Instead, the increase could be accounted for by the combined effect of the altered ORF1p amino acid sequence and the deletion of the GRR from the 3 prime untranslated region. In other words, two specific sequence changes introduced as side effects of the cloning strategy, rather than the deliberate codon reengineering, were responsible for the phenotype that the field had attributed to adenosine depletion.

This finding matters because smL1 and related synthetic constructs are workhorses in retrotransposition research. They are used to measure L1 activity in cell culture, to screen for host factors that suppress or promote retrotransposition, and to model how new L1 insertions arise. If the interpretive framework around these tools is built on an incorrect causal story, downstream conclusions about L1 biology could be skewed. The new results suggest that adenosine richness per se is not the dominant constraint on L1 expression that it was once thought to be, and that researchers comparing native and synthetic elements need to control carefully for every sequence difference, not just the intended ones.

The role of the GRR proved to be particularly interesting. When the researchers reintroduced the guanosine-rich region into natural mouse L1 elements, they found that its effect on retrotransposition depended strongly on where in the element it was placed. This position-dependent inhibition did not operate through premature polyadenylation, the mechanism by which some 3 prime untranslated region sequences truncate transcripts and thereby cripple expression. Nor did the GRR change the structure of the L1 insertions that were recovered, indicating that the element’s fundamental retrotransposition mechanism remained intact. Instead, the evidence pointed to a different explanation: the GRR modulates the abundance of the L1 ORF1p protein. Less ORF1p means fewer functional ribonucleoprotein particles, and therefore less retrotransposition.

The contrast between natural and synthetic elements sharpened this picture. While the GRR suppressed retrotransposition of natural mouse L1 in a position-dependent fashion, smL1 was essentially refractory to this inhibition. The authors attribute this resistance to the highly efficient protein expression driven by the synthetic open reading frames, which apparently produces enough ORF1p to overwhelm or bypass the dampening effect of the GRR. Taken together, these observations suggest a model in which the G-quadruplex-forming sequence acts as a rheostat on L1 protein output, and in which synthetic elements escape this regulation because their recoded coding sequences translate so efficiently. The precise molecular mechanism linking the GRR to ORF1p abundance remains an open question, and the authors explicitly call for further investigation of the relationship between this sequence and L1 protein expression.

Beyond its implications for mouse L1, the study carries a broader methodological lesson for the retrotransposon field and for genome engineering generally. Synthetic recoding is a powerful strategy, but every nucleotide change carries the potential for unintended functional consequences, whether through altered protein sequence, altered RNA structure, altered transcript processing or altered translation kinetics. The 3 prime untranslated region, in particular, has emerged as a hotspot of regulatory information, and deleting or rearranging it can produce phenotypes that are easily misattributed to coding sequence changes. The authors demonstrate that the arrangement of L1 nucleotide sequences within reporter constructs can itself influence L1 expression and engineered retrotransposition efficiency, a caution that applies to any experimental system in which retroelements are rebuilt for laboratory use.

The work also reframes a long-standing question about L1 evolution. If adenosine-rich coding sequence is not the primary brake on L1 expression, why do L1 open reading frames remain so A-rich? One possibility raised by the new findings is that regulatory sequences such as the GRR, rather than codon composition, tune L1 activity in its natural context, allowing host genomes and the elements themselves to modulate retrotransposition without sacrificing protein function. Understanding how G-quadruplex-forming regions in retrotransposon transcripts control protein output could illuminate not only L1 biology but also the broader interplay between RNA structure and gene expression in mammalian cells. For now, the study stands as a reminder that in retrotransposition, as in genome biology at large, the sequence you did not intend to change can matter as much as the one you did.

Subject of Research: Sequence determinants of engineered LINE-1 retrotransposition efficiency in mouse

Article Title: Revisiting the impact of synthetic ORF sequences on engineered LINE-1 retrotransposition

Article References: Chan, D., Workman, S., Smits, N., Carleton, A. L., Gerdes, P., Han, J. S., Boeke, J. D., Faulkner, G. J., & Richardson, S. R. (2026). Revisiting the impact of synthetic ORF sequences on engineered LINE-1 retrotransposition. PLOS Genetics, 22(10), e1012340. https://doi.org/10.1371/journal.pgen.1012340

Image Credits: AI Generated

DOI: 10.1371/journal.pgen.1012340

Keywords: LINE-1, retrotransposition, transposable elements, synthetic biology, G-quadruplex, ORF1p, 3'UTR, mouse genome, gene expression, PLOS Genetics, codon optimization, genome engineering