The maternal KRAB-ZFP ZFPOBI1 reveals structural constraints governing ERV transcriptional co-option in mouse oocytes

Transposable elements (TEs) constitute a major fraction of mammalian genomes and play key roles in gene regulation, particularly during early development. Endogenous retroviruses (ERVs) are highly active in oocytes and early embryos, where their long terminal repeats (LTRs) can act as alternative promoters to generate LTR-initiated transcripts (LITs). Krüppel-associated box zinc finger proteins (KRAB-ZFPs) on the other hand repress TE activity in a sequence-specific manner through recruitment of the co-repressor TRIM28. Here, we identify the mouse KRAB-ZFP ZFPOBI1 as a previously uncharacterized, maternally expressed KRAB-ZFP that selectively targets the RLTR10 LTR subfamilies of the ERVK class. ZFPOBI1 binding is associated with robust TRIM28 recruitment and more modest changes in H3K9me3 enrichment at RLTR10 elements in mouse embryonic stem cells, consistent with canonical KRAB-ZFP-function. In oocytes, we show that RLTR10 elements contribute to LIT formation in a structure-dependent manner. While LTRs serve as transcriptional start sites, efficient splicing into downstream exons predominantly occurs via internal (-int) ERV sequences, indicating a functional separation of transcription initiation and RNA processing. Maternal deletion of ZfpObi1 results in upregulation of a subset of RLTR10-driven LITs, demonstrating a role for ZFPOBI1 in restraining ERV-derived transcription. Notably, full-length RLTR10 elements are subject to additional KRAB-ZFP targeting at internal regions, suggesting that their repression is achieved through multilayered control. Consistent with this, the limited extent of transcriptional deregulation in ZfpObi1-deficient oocytes indicates partial functional redundancy within the KRAB-ZFP family. Together, our findings identify ZFPOBI1 as a regulator of RLTR10 elements and reveal how ERV structural organization constrains both transcriptional co-option and its epigenetic control in the oocyte transcriptome.