Both prokaryotes and eukaryotes produce an immune response against plasmids with 5'-GTTTGTT-3'.

In the evolutionary "arms race" from prokaryotes to eukaryotes, some memories of foreign DNA have been conserved for defensive purposes. Shortly after invasion by the plasmid, pEGFP-N1, the conserved the defense gene, isg15, was activated in the zebrafish zygote and in mammalian cells. Based on the sequence similarity, we found three virus-derived sequences in pEGFP-N1 which share the 5'-GTTTGTT-3' core sequence, an epigenetic factor leading to increased expression of isg15. Mutation of the core sequence greatly reduces the degradation rate of the plasmid in E. coli cells or zebrafish embryos. We conclude that a conserved defense response, common to both eukaryotic and prokaryotic cells, allows identification and degradation of plasmids containing 5'-GTTTGTT-3'.