Polycistronic vs Monocistronic
Polycistronic mRNA refers to a messenger RNA which encodes two or more proteins. Polycistronic messenger RNAs participates in the process of protein synthesis (translation) in prokaryotes.
Many bacterial mRNAs are fully functional in their primary composition transcripts meaning that they do not require any posttranscriptional modifications. In prokaryotes an mRNA molecule usually contains multiple open reading frames (ORFs) each corresponding to a single gene transcript – (polycistronic mRNA, also the “polygenic mRNA” synonym is used) or, more rarely, the transcript of a single gene (monocistronic or monogenic mRNA). In polycistronic mRNAs the information for each ORF is translated into a polypeptide.
Polycistronic mRNA structure
A polycistronic mRNA contains the following distinct regulatory and functional regions:
- a 5’ leader sequence;
- coding regions (structural gene transcripts), each beginning with a translation initiation codon and consisting of a linear sequence of codons;
- (sometimes) a 3’-terminal trailer;
- intercistronic regions The coding regions may or may not be separated by non-coding sequence regions.
The messenger may later be cleaved into individual messages, each of which is translated into a single protein. These polypeptide molecules usually have a related function – they could be subunits which build a final complex protein or could participate in a common biological process. their coding sequence is grouped and regulated together in a regulatory region, containing a promoter and an operator. A classical example is the lac operon (q.v.) of E. coli which generates a polycistronic mRNA. In most of the bacteria and archea, the mRNAs are found to be with polycistronic structure.
Rarely the terms dicistronic or bicistronic could be used in scientific literature, which in this case refer to an mRNA that encodes only two proteins.
Viral Polycistronic Messengers
It is important to mention that in some ribonucleic acid viruses could be found polycistronic messengers that contain the amino acid sequence for several proteins.