POLYADENYLATION OF MESSENGER-RNA IN PROKARYOTES

The 3'-ends of both prokaryotic and eukaryotic mRNA are polyadenylated , but the poly(A) tracts of prokaryotic mRNA are generally shorter, ra nging from 15 to 60 adenylate residues and associated with only 2-60% of the molecules of a given mRNA species. The sites of polyadenylation of bacterial mRNA are diverse and include the 3'-ends of primary tran scripts, the sites of endonucleolytic processing in the 3' untranslate d and intercistronic regions, and sites within the coding regions of m RNA degradation products. The diversity of polyadenylation sites sugge sts that mRNA polyadenylation in prokaryotes is a relatively indiscrim inate process that can occur at all mRNA's 3'-ends and does not requir e specific consensus sequences as in eukaryotes. Two poly(A) polymeras es have been identified in Escherichia coli. They are encoded by unlin ked genes, neither of which is essential for growth, suggesting signif icant functional overlap. Polyadenylation promotes the degradation of a regulatory RNA that inhibits the replication of bacterial plasmids a nd may play a similar role in the degradation of mRNA. However, under certain conditions, poly(A) tracts may lead to mRNA stabilization. The ir ability to bind S1 ribosomal protein suggests that poly(A) tracts m ay also play a role in mRNA translation.