The Bacterial Operon The concept of an operon is central to hypotheses about bacterial transcriptional regulation. Bacterial genes are often arranged so that more than one gene can be transcribed into the same polycistronic mRNA. In such cases. the genes are said to be cotranscribed. Cotranscription of more than one gene into a polycistronic mRNA Seems to be unique to bacteria and their phages and affects the types of translational initiation regions TIR)used on the mRNA. In eukaryotes, generally the first AUg codon in an mrNa is used to initiate translation so that only one polypeptide can be encoded by each mRNA However, bacterial TIRs are much more complex. Shine Dalgarno and other sequences help define the tir because of its distinct structure. a bacterial TIR will be recognized wherever it appears in the mRNA, and more than one tir can be recognized in the same mRNA• The Bacterial Operon • The concept of an operon is central to hypotheses about bacterial transcriptional regulation. Bacterial genes are often arranged so that more than one gene can be transcribed into the same polycistronic mRNA. In such cases, the genes are said to be cotranscribed. Cotranscription of more than one gene into a polycistronic mRNA seems to be unique to bacteria and their phages and affects the types of translational initiation regions (TIR) used on the mRNA. In eukaryotes, generally the first AUG codon in an mRNA is used to initiate translation so that only one polypeptide can be encoded by each mRNA. However, bacterial TIRs are much more complex. Shine￾Dalgarno and other sequences help define the TIR. Because of its distinct structure, a bacterial TIR will be recognized wherever it appears in the mRNA, and more than one TIR can be recognized in the same mRNA