When transcription of bacterial rRNAs and tRNAs is completed they are immediately ready for use in translation. No additional processing takes place. Translation of bacterial mRNAs can begin even before transcription is completed due to the lack of the nuclear-cytoplasmic separation that exists in eukaryotes. The ability to initiate translation of prokaryotic RNAs while transcription is still in progress affords a unique opportunity for regulating the transcription of certain genes. An additional feature of bacterial mRNAs is that most are polycistronic. This means that multiple polypeptides can be synthesized from a single primary transcript. Polycistronic mRNAs are very rare in eukaryotic cells but have been identified. The mitochondrial genomes in mammals and the slime mold, Dictyostelium discoideum , encode polycistronic mRNAs that are processed into primarily mono-, di-, and tricistronic transcripts. In addition, several viruses encode polycistronic RNAs.