RNA, Caps, and Polyadenylate Tails

Inducible operons 1642 4. Messenger RNA, Caps, and Polyadenylate Tails... 

Eukaryotic mRNAs contain a 507-methylguanosine cap and a 30 polyadenylate tail, both of which are added by specific enzymes localized to the nucleus. Alternative RNA splicing and polyadenylation contribute to transcript heterogeneity and specificity, and can result in the synthesis of cell-specific proteins. 

It is also unclear whether archaeal mRNAs can be cotranscriptionally or post-transcriptionally modified. Polyadenylated RNAs have been identified in several archaea [21-23], the poIy(A) tails being quite short in methanogens [22] and rather long in the halophiles and the crenarchaeota [21-23]. However, evidence that they are indeed mRNAs is lacking. In any event Poly(A)+ RNAs are not capped in Methanococcus vannielii [22]. 

Poliovirus RNA in many respects looks like a typical eukaryotic mRNA with a polyadenylated 30 tail and a protected 50 terminus. However, the poly (A) tail is not added by the action of poly (A) polymerase, but instead is encoded in the viral genome. In addition, the 50 end does not contain a 7-methylguanosine cap, but rather a covalently attached viral protein called VPg that has a phosphodiester linkage between a tyrosine residue in VPg and a uridine in the viral RNA. 

Messenger RNA polyadenylation is an important aspect of gene expression in eukaryotes. The process itself is physically and temporally coupled with several other steps in gene expression, ranging from the initiation of transcription to the export of the mature mRNA to the cytoplasm. In the cytoplasm, the poly(A) tail constitutes an essential cis-element that enables the translation of the mRNA, in cooperation with the 5 -cap. 

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