The structures of mouse and human L1 elements reflect their insertion mechanism

Area of Science:

• Genomics
• Molecular Biology
• Evolutionary Biology

Background:

• L1 elements are abundant, interspersed DNA sequences in mammalian genomes.
• L1 achieves high copy numbers through retrotransposition, a process involving reverse transcription.
• L1 insertion into DNA typically occurs via target-site primed reverse transcription (TPRT).

Purpose of the Study:

• To investigate the mechanism behind 5' truncations observed in L1 retrotransposition.
• To challenge the assumption that truncations are solely due to poor reverse transcriptase processivity.
• To elucidate the role of target site sequence in L1 insertion and truncation.

Main Methods:

• Analysis of young L1 elements from human and mouse genomes.
• Examination of the relationship between L1 element truncation sites and genomic target sequences.
• Comparative analysis of L1 elements with varying degrees of 5' truncation.

Main Results:

• The majority of young L1 elements are truncated at sequences capable of basepairing with the target site.
• Truncation is more likely when L1 sequence can basepair with the target site sequence.
• Evidence supports a model involving two sequential TPRT reactions, with homology playing a role in the second.

Conclusions:

• L1 element truncation is influenced by target site sequence homology, not just reverse transcriptase processivity.
• A model of sequential TPRT reactions, involving homology-dependent steps, explains L1 insertion.
• 5' truncation may be an evolutionary strategy for L1 to evade host suppression mechanisms.