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Bacterial Artificial Chromosomes Establish Replication Timing and Sub-Nuclear Compartment De Novo as Extra-Chromosomal Vectors

The role of DNA sequence in determining replication timing (RT) and chromatin higher order organization remains elusive. To address this question, we have developed an extra-chromosomal replication system consisting of ~200kb human bacteria artificial chromosomes (BACs) modified with Epstein-Barr virus (EBV) replication origin elements (E-BACs). E-BACs were stably maintained as autonomous mini-chromosomes in both HeLa and human induced pluripotent stem cells (hiPSCs) and established distinct RT patterns. An E-BAC harboring an early replicating chromosomal region replicated early during S phase, while E-BACs derived from RT transition regions (TTRs) and late replicating regions replicated in mid to late S phase. Analysis of E-BAC interactions with cellular chromatin (4C-seq) revealed that the early replicating E-BAC interacted broadly throughout the genome and preferentially with the early replicating compartment of the nucleus. In contrast, mid- to late-replicating E-BACs interacted with more specific late replicating chromosomal segments, some of which were shared between different E-BACs. Together, we describe a versatile system in which to study the structure and function of chromosomal segments that are stably maintained separately from the influence of cellular chromosome context.