Question

Discuss the pros and cons of polycistronic mRNA transcription.

Answer 1

As the name suggests, a polycistronic mRNA has many cistrons or open reading frame, each getting translated in to a polypeptide. Generally polypeptides synthesiszed from a polycistronic mRNA are functionally related. They are generally found in prokaryotes, chloroplast and mitochondrial genome.

Pros:

Since polycistronic mRNA carries information for many proteins in a single mRNA molecule, single round of transcription is required for that much of proteins. In contrast monocistronic mRNA has information for only one protein, and for every protein, separate mRNA has  to be synthesized and thus that many round of transcription is required. Thus, polycistronic mRNA transcription is energy saving process (due to lesser number of trancription cycle).

Polycistronic mRNA usually carry information for two or more proteins which are functionally related. They may be related to same metabolic pathway (for example Lac operon with three structural genes involved in lactose metabolism) or are subunits of an enzyme complex. This ensures availability of all components of a metabolic pathway at any given time. For example, When Lac operon is active, all the three polypeptide will be available.

Since polycistronic mRNAs are found in prokaryotes, where the genome size is smaller as compared to eukaryotes and most part of genome is coding. Only a small fraction of prokaryotic genome is non-coding and have regulatory function. Since a small sized prokaryotic cell can not afford to contain many regulatory DNA regions (non-coding regions), operon system provides advantage.  Operons are set of genes which are under the control of single promoter. This ensures more number of functional genes with lesser number of promoters in small sized prokaryotic genome.

Unlike monocistronic mRNA, which undergo post-transcriptional modifications, polycistronic mRNAs do not undergo energy intensive post-transcription modifications. This further ensures energy cost effective process of cell.

Cons

Polycistronic mRNAs also have many disadvantages as well. Since polycistronic mRNAs are synthesized from operon which are under the control of a single promoter, one of the major cons of polycistronic mRNA is that all the genes are turned ON or OFF together.

If a mutation occurs in one promoter, the whole operon (containing information for many proteins) will not be transcribed. However in a monocistronic mRNA, mutation in one promoter will effect only one protein. This is a major disadvantage of polycistronic mRNAs.