polycistronic mRNA

Think of a printed program for a concert that lists several pieces back to back on one sheet — symphony, then concerto, then encore — so a single page tells the orchestra everything to play that night. A polycistronic mRNA is a molecular version of that single sheet: one RNA molecule carrying the instructions for several different proteins, lined up one after another.

When a bacterium transcribes an operon, RNA polymerase runs through all the clustered genes in one go and produces a single long mRNA. That one molecule contains, in a row, the coding regions for each protein, each with its own start and stop signals for the ribosome. Bacterial ribosomes can latch on at internal sites along the message — guided by a short sequence (the Shine-Dalgarno sequence) just ahead of each protein's start codon — so several ribosomes can independently translate the different coding stretches of the same RNA. The word comes from 'cistron', an old term for a gene; 'poly' means many. A message coding for just one protein, by contrast, is monocistronic.

Polycistronic mRNA is how the operon delivers on its promise: transcribe one stretch of DNA, and you get the instructions for a whole team of related proteins in proportions the cell can tune. It is the normal form of bacterial messenger RNA for grouped genes. Eukaryotic mRNAs are almost always monocistronic — each carries one protein's worth of instructions and ribosomes generally begin only at the first start codon — which is one of the deepest practical differences between bacterial and eukaryotic gene expression.