Polycistronie

Polycistronie is the organization of a single transcription unit that yields a polycistronic messenger RNA encoding several distinct proteins. This arrangement is common in bacteria and archaea, and it also occurs in the chloroplasts and mitochondria of some plants and other organisms. Genes contained in a polycistronic transcript are usually functionally related and are often arranged together in an operon, which has a shared promoter and terminator that initiate and terminate transcription.

In polycistronic transcripts, translation typically proceeds with ribosomes initiating at the start codon of each gene, using gene-specific ribosome binding sites such as the Shine-Dalgarno sequence in bacteria. As a result, one mRNA can produce multiple polypeptides. The operon structure enables coordinated regulation of the encoded proteins, allowing the cell to adjust expression of a whole pathway in response to environmental cues. Classic examples include the lac operon and the trp operon in Escherichia coli.

Regulation and organization of polycistronic transcripts involve promoter strength, operator and repressor interactions, and transcription termination, as well as translational control mechanisms such as translational coupling and internal ribosome entry sites in some systems. Some operons contain internal promoters that generate monocistronic transcripts for specific genes, adding flexibility to regulation.

Evolution and significance: Polycistrony provides coordinated expression of functionally related genes, which is advantageous in organisms with compact genomes and simpler regulatory networks, especially among prokaryotes and organellar genomes. In contrast, most eukaryotic nuclear genes are monocistronic. The term polycistronie is often used in French, while English use tends to be polycistronic or polycistrony.