polycistronic

Polycistronic describes a nucleic acid—typically an mRNA—that contains the coding information for more than one polypeptide. In this arrangement, several distinct coding sequences, or cistrons, are transcribed as a single RNA molecule. Polycistronic transcripts are most common in living organisms whose gene expression is organized into operons or polycistronic units.

In bacteria and many archaea, polycistronic mRNAs are the norm. Genes in an operon are co-transcribed from a single promoter, and translation often initiates at multiple sites within the same transcript. Each downstream cistron usually has its own ribosome binding site, such as a Shine-Dalgarno sequence, allowing independent initiation of translation and sometimes translational coupling between adjacent genes.

In eukaryotes, monocistronic mRNAs that encode a single polypeptide are the standard. However, polycistronic transcripts occur in certain contexts, notably in organelles such as mitochondria and chloroplasts, where long polycistronic precursors are processed into separate functional RNAs. Some eukaryotic systems—such as the transcription of long polycistronic pre-mRNAs in trypanosomes and the operon-like clustering seen in nematodes—employ special RNA processing steps (for example trans-splicing) to yield individual mRNAs. In biotechnology, polycistronic expression in eukaryotes can be achieved using internal ribosome entry sites or 2A peptide sequences, enabling the production of multiple proteins from a single transcript.

Overall, polycistronic organization reflects a strategy to coordinate expression of functionally related genes, with mechanisms adapted to the cellular translation and RNA processing environment.