Figure 5.

ERV sequences that escape TRIM28-mediated repression can act as activators during embryogenesis. Lentiviral transgenesis was performed with an empty PGK-GFP vector (PGK-GFP control, upper panels), or with the same vector including either an IAP4 (TRIM28-sensitive IAP-PGK-GFP, middle panels) or an IAP1 (TRIM28-resistant IAP-PGK-GFP, lower panels) sequence cloned antisense upstream of the PGK promoter. At E13, embryos were scored for GFP expression and vector copy numbers. For the PGK-GFP control, 13/29 embryos were green. For the TRIM28-sensitive IAP-PGK-GFP, 4/19 embryos were green (all with copy numbers above 16), and 4/19 pale green (including numbers 3 and 4 in this figure). For the TRIM28-resistant IAP-PGK-GFP, 12/17 embryos were green (including one with a copy number above 10), and 2/17 pale green (with copy numbers of 0.95 and 0.89). Embryos with similar copy numbers per vector group are shown in each column with increasing copy numbers by row. Vectors were injected twice with similar results. In one experiment, MEFs were derived from embryos to verify that microscopy differences were reproduced by flow cytometry (data not shown).