IMMUNOFLUORESCENT LOCALIZATION OF ACTIN IN RELATION TO TRANSCRIPTION SITES IN MOUSE PRONUCLEI

Previous biochemical and morphological studies have shown the presence of actin in the nucleus of different cell types where its role remain s unclear. In this work, through fluorescence microscopy we studied th e localization of actin in the nuclei of early mouse embryos with part icular attention to its possible involvement in the onset of transcrip tion occurring at the late one-cell stage. Fluorescent labelling of em bryo sections showed that nuclear actin is abundant, in a non-filament ous state, in the whole nucleoplasm excluding the nucleolar precursor bodies. Immunofluorescence on permeabilized embryos revealed that inso luble nuclear actin accumulates in a few large aggregates in transcrip tionally inert early one-cell embryos and progressively redistributes into many small aggregates in transcriptionally active late one-cell e mbryos. Interestingly, these actin aggregates clearly colocalize with transcription sites. Treatment of late one-cell embryos with cytochala sin D induces the formation of actin bundles network in the nucleoplas m but has no apparent effect on the transcriptional activity. In addit ion, the inhibition of transcription by oc-amanitin does not modify th e nuclear actin distribution. Hence, there does not appear to be a dir ect causal relationship between transcriptional activity and nuclear a ctin organization at the one-cell stage although nuclear actin aggrega tes appear associated with transcription sites. (C) 1998 Wiley-Liss, I nc.