Preferential nuclear location of a transgene does not depend on its transcriptional activity during early mouse development.

Changes in chromatin structure play an important role in regulation of the HSP70.1 gene during mouse preimplantation development. Using in situ PCR we have now examined whether the spatial organization of an HSP70.1 luciferase transgene within the nucleus is also a factor in regulating its expression. The transgene showed a preferential localization towards the nuclear periphery throughout preimplantation development. This preferential location was independent of the level of constitutive activity of the transgene and did not change when transgene expression was induced through core histone hyperacetylation at the eight-cell stage or by heat shock in blastocysts. In contrast, at the two-cell stage, when embryos are unable to continue development after heat shock, thermal stress provoked a significant disruption of the nuclear location of the transgene. These results do not agree with a recent model of embryonic genome activation in mice which hypothesizes that directed, active movement of DNA within the nucleus is a determinant factor in establishing early patterns of gene expression. Instead, they are consistent with models proposing that chromatin segments are restricted to nuclear subregions, but that they remain free to undergo substantial Brownian motion.