Cleavage of poly(A) tails on the 3 '-end of RNA by ribonuclease E of Escherichia coli

RNase E initiates the decay of Escherichia coli RNAs by cutting them intern ally near their 5'-end and is a component of the RNA degradosome complex, w hich also contains the 3'-exonuclease PNPase, Recently, RNase E has been sh own to be able to remove poly(A) tails by what has been described as an exo nucleolytic process that can be blocked by the presence of a phosphate grou p on the 3'-end of the RNA. We show here, however, that poly(A) tail remova l by RNase E is in fact an endonucleolytic process that is regulated by the phosphorylation status at the 5'- but not the 3'-end of RNA. The rate of p oly(A) tail removal by RNase E was found to be 30-fold greater when the 5'- terminus of RNA substrates was converted from a triphosphate to monophospha te group, This finding prompted us to re-analyse the contributions of the r ibonucleolytic activities within the degradosome to 3' attack since previou s studies had only used substrates that had a triphosphate group on their 5 '-end. Our results indicate that RNase E associated with the degradosome ma y contribute to the removal of poly(A) tails from 5'-monophosphorylated RNA s, but this is only likely to be significant should their attack by PNPase be blocked.