Coupled transcription and translation within nuclei of mammalian cells

It is widely assumed that the vital processes of transcription and translat ion are spatially separated in eukaryotes and that no translation occurs in nuclei. We localized translation sites by incubating permeabilized mammali an cells with [H-3]lysine or lysyl-transfer RNA tagged with biotin or BODIP Y; although most nascent polypeptides were cytoplasmic, some were found in discrete nuclear sites known as transcription "factories." Some of this nuc lear translation also depends on concurrent transcription by RNA polymerase II. This coupling is simply explained if nuclear ribosomes translate nasce nt transcripts as those transcripts emerge from still-engaged RNA polymeras es, much as they do in bacteria.