In prokaryotes, the relation between the primary transcript and the mRNA is as simple as can be: The primary transcript is the mRNA. Even as the 3′ end of the primary transcript is still being synthesized, ribosomes bind with special sequences near its 5′ end and begin the process of protein synthesis (Fig. 3.20). This intimate connection between transcription and translation can take place because prokaryotes have no nuclear envelope to spatially separate transcription from translation; the two processes are coupled, which means that they are connected in space and time.

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Primary transcripts for protein-coding genes in prokaryotes have another feature not shared with those in eukaryotes: They often contain the genetic information for the synthesis of two or more different proteins, usually proteins that code for successive steps in the biochemical reactions that produce small molecules needed for growth, or successive steps needed to break down a small molecule used for nutrients or energy. Molecules of mRNA that code for multiple proteins are known as polycistronic mRNA because the term “cistron” was once widely used to refer to a protein-coding sequence in a gene.