Question

Why do bacterial mRNAs, but not eukaryotic mRNAs, require a Shine-Dalgarno sequence?

A. mRNA splicing would remove Shine-Dalgarno sequences.

B. Eukaryotic mRNAs must exit the nucleus. C. Bacterial mRNAs are shorter lived. D. The eukaryotic poly-A tail serves the same function. E. Bacterial mRNAs are often polycistronic.

Answer 1

E. Bacterial mRNAs are often polycistronic

Shine-Dalgarno sequence is a regulatory unit in the translation process in the bacteria. It is located typically around the -7 to -4 position of the translational start codon and has the sequence AGGAGG. Now, the way start codon is determined in bacteria is different in prokaryotes than in the eukaryotes. In bacteria, there are no 5' caps unlike the eukaryotes and so the assembly of ribosomes differs in each case.

Since the ribosomes in the prokaryotes can be assembled once a Shine-Dalgarno sequence is encountered and given the knowledge that bacterial mRNA can have multiple start sites, one mRNA can code for multiple polypeptides. Hence, they are called polycistronic mRNA (a sequence of nucleotides that code for a polypeptide is called cistron).