How does a polycistronic mRNA operate in prokaryotes?

A polycistronic mRNA operates in prokaryotes by encoding multiple proteins within a single mRNA molecule.

In prokaryotes, the process of transcription can lead to the formation of a single mRNA molecule that carries the genetic information for more than one protein. This type of mRNA is known as polycistronic mRNA. The term 'polycistronic' refers to the fact that this mRNA contains multiple 'cistrons', or coding sequences, each of which corresponds to a different protein.

The operation of polycistronic mRNA is closely linked to the way in which prokaryotes organise their genes. Unlike eukaryotes, where genes are typically scattered throughout the genome, prokaryotes often group related genes together into units known as operons. Each operon is transcribed as a single piece of mRNA, which is then translated into multiple proteins. This allows prokaryotes to coordinate the production of proteins that are involved in the same cellular process.

The process begins with the binding of RNA polymerase to a specific sequence of DNA known as the promoter. This signals the start of transcription. The RNA polymerase then moves along the DNA, synthesising a strand of mRNA that is complementary to the DNA sequence. When the RNA polymerase reaches a sequence known as the terminator, it stops transcription and releases the newly formed mRNA molecule.

This mRNA molecule contains the genetic information for multiple proteins, each encoded by a different cistron. These cistrons are separated by sequences known as intercistronic regions, which help to ensure that each cistron is translated independently. The ribosome binds to the mRNA at the start of a cistron, translates the genetic information into a protein, and then releases the protein before moving on to the next cistron.

In this way, a single polycistronic mRNA molecule can direct the synthesis of multiple proteins. This is a highly efficient system, as it allows prokaryotes to produce all the proteins required for a particular process at the same time, using a single mRNA molecule. It also provides a mechanism for regulating the production of these proteins, as the transcription of the entire operon can be controlled by a single promoter.