Stable Episomal Maintenance of Yeast Artificial Chromosomes in Human Cells

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Abstract
Plasmids carrying the Epstein-Barr virus origin of plasmid replication (oriP) have been shown to replicate autonomously in latently infected human cells (J. Yates, N. Warren, D. Reisman, and B. Sugden, Proc. Natl. Acad. Sci. USA 81:3806-3810, 1984). We demonstrate that addition of this domain is sufficient for stable episomal maintenance of yeast artificial chromosomes (YACs), up to at least 660 kb, in human cells expressing the viral protein EBNA-1. To better approximate the latent viral genome, YACs were circularized before addition of the oriP domain by homologous recombination in yeast cells. The resulting OriPYACs were maintained as extrachromosomal molecules over long periods in selection; a 90-kb OriPYAC was unrearranged in all cell lines analyzed, whereas the intact form of a 660-kb molecule was present in two of three cell lines. The molecules were also relatively stable in the absence of selection. This finding indicates that the oriP-EBNA-1 interaction is sufficient to stabilize episomal molecules of at least 660 kb and that such elements do not undergo rearrangements over time. Fluorescence in situ hybridization analysis demonstrated a close association of OriPYACs, some of which were visible as pairs, with host cell chromosomes, suggesting that the episomes replicate once per cell cycle and that stability is achieved by attachment to host chromosomes, as suggested for the viral genome. The wide availability of YAC libraries, the ease of manipulation of cloned sequences in yeast cells, and the episomal stability make OriPYACs ideal for studying gene function and control of gene expression.