In bacteria, it is common to produce polycistronic messages; in eukaryotes it is less common. Describe how polycistronic mRNAs are made and processed in prokaryotes and eukaryotes. How might polycistronic mRNA molecules be an advantage for eukaryotes?
What will be an ideal response?
Answer: In prokaryotes, polycistronic mRNAs typically encode all the information associated with products required for a particular pathway that is organized as an operon. This arrangement allows for efficiency and an additional mechanism for translational regulation.
In eukaryotes, one of two mechanisms occurs with regard to polycistronic messages. In some organisms the pre-RNA is made in a polycistronic fashion. It is then processed into monocistronic messages for translation. In other organisms, dicistronic messages are produced and translated together.
In eukaryotes, there is the luxury of the nucleus, which partitions transcription and translation. This allows for monocistronic messages to be advantageous. However, C. elegans is one of the few eukaryotes that has 25% of its genes organized in an operon fashion. Therefore, it would be interesting to hypothesize that the pre-mRNA would also be transcribed in a similar fashion and then divided into monocistronic mRNAs for that operon. As for dicistronic messages, one may hypothesize that the two products allow for other processing, such as alternate products or products that give an advantage of being translated together.
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