Coupled transcription–translation occurs in
Because bacteria possess no nuclear membrane separating transcription and translation compartments, the two processes occur simultaneously on nascent RNA in coupled manner. As RNA polymerase elongates 5' to 3', the emerging 5' end, once the Shine-Dalgarno sequence exits polymerase channel, is immediately bound by 30S subunits and initiates translation while transcription proceeds downstream, forming polysomes trailing behind polymerase. This coupling allows attenuation control in trp operon and rapid adaptation to environmental changes. Eukaryotes localize transcription within the nucleus, requiring capping, splicing, polyadenylation, and export before cytoplasmic translation, precluding direct physical coupling.
Ref: Lodish 8th Molecular Cell Biology Fig 4-6; NCBI Bookshelf NBK21429 - transcription-translation coupling in prokaryotes via lack of nucleus