Polycistronic mRNA

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Pol y a d e ny l a t i o n

is reduced when they are deadenylated. It is still not understood how the polyadenylation or deadenylation is related to the control of the translational utilization of the mRNA. More recently, RNAs with poly(A) tails have been found in bacteria. The poly(A) tails have been found to reduce the stability of regulatory plasmid RNAs and mRNAs, but the mechanisms are not clear. Finally, it has been shown in bacteria that stable RNAs such as tRNA, tmRNA, and 4.5S, 6S, and ribosomal RNAs can be found in polyadenylated forms. These findings indicate that polyadenylation is not unique to mRNA and that it serves a more general function in RNA metabolism. See also: Messenger RNA (mRNA)

Polyadenylation Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1963

Polyadenylation is the addition of a sequence of polyadenylic acid to the 30 end of a eukaryotic RNA following its transcription. See also: Transcription

Polycistronic mRNA J Parker Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1008

A polycistronic messenger RNA (mRNA) is a single RNA molecule encodes more than one protein by virtue of containing information from two or more sequential, functional open reading frames (ORFs). Therefore, each protein is produced independently. (This is in contrast to polyproteins, which are synthesized as a single polypeptide and then posttranslationally processed into a number of different functional proteins). Polycistronic mRNA is found almost exclusively in prokaryotes, since the mechanism of translational initiation by prokaryotic ribosomes allows the ribosome to readily initiate at start codons located internally on mRNA. Most operons yielding polycistronic mRNA contain genes whose synthesis, or whose efficient function,

would seem to require coordinate regulation, for example, those encoding enzymes in a biosynthetic pathway. There might be a natural advantage in keeping such genes together in organisms where polycistronic mRNA can be translated. When comparing sequenced genomes of widely divergent organisms, the most highly conserved polycistronic mRNAs seem to be those from operons encoding certain ribosomal proteins. Typically, each gene on a polycistronic mRNA contains its own Shine±Dalgarno sequence, a sequence that is involved in initiation by prokaryotic ribosomes, and therefore each gene can be translated independently. However, there are instances where translational initiation at one site is dependent on translation of some other region of the mRNA. In some cases, changes in the secondary structure of the polycistronic mRNA `induce' initiation sites for other proteins, e.g., in the small RNA bacteriophage MS2 the translation of the replicase gene is dependent on translation of the coat protein gene and concomitant disruption of secondary structure. In addition, translational reinitiation has been observed in cases where the stop codon of the upstream gene is close to the start codon of the next gene. Here the 30S ribosomal subunit apparently does not dissociate from the message before reinitiating at a nearby site. Such reinitiation can make the efficiency of the downstream initiation site much greater than if the site was on monocistronic mRNA. For this reason, some cloning vectors designed to yield very high levels of expression of a gene have incorporated into them a small, upstream, functional ORF. Translational coupling is a type of regulation where the translation of a distal gene is very highly dependent on translation of the gene immediately upstream in the polycistronic messages and either of the two mechanisms mentioned above could be involved. Although polycistronic mRNA is typical of prokaryotes, most messages produced in these organisms is from transcriptional units that yield monocistronic mRNA. In Escherichia coli, less than 30% of the mRNA is polycistronic, and polycistronic mRNA seems to be even less common than this in the Archaea. See also: Cistron; Open Reading Frame; Operon; Translation

Polygenes See: Complex Traits