Messenger RNA molecules (mRNAs) are single-stranded RNAs carrying the message that is to be translated into protein. In prokaryotic cells, messages for several proteins may be carried on a single mRNA (called polycistronic messages). Polycistronic messages arise because prokaryotic genes are organized into operons (see here). Figure 27.5 depicts the lac operon from E. coli. Each of the messages for a single protein on the mRNA is contained in a single "open reading frame". In this case, the open reading frames correspond to the lac z, y, and a genes.

An open reading frame is a region of the nucleic acid sequence uninterrupted by start or stop codons. In polycistronic prokaryotic mRNA, the open reading frames are bounded by start and stop codons.

Just before the AUG start codon, prokaryotic mRNAs have a common sequence called the Shine-Dalgarno sequence. Some variations on the sequence are shown in Table 27.3. The Shine-Dalgarno sequences are complementary to a sequence contained in the 16S ribosomal RNA (rRNA) (Table 27.3) and help align the mRNA with the ribosome to properly orient the molecules for translation initiation.

Some mRNA molecules are able to base-pair within themselves due to intramolecular complementarity and because mRNA molecules are single-stranded. As a result, some mRNA molecules can form the three-dimensional secondary and tertiary structures, which can play a role in regulation of the relative productions of various protein products.