Distinct Requirements for 5'-Monophosphate-assisted RNA Cleavage by Escherichia coli RNase E and RNase G.

RNase E and RNase G are homologous endonucleases that play important roles in RNA processing and decay in Escherichia coli and related bacterial species. Rapid mRNA degradation is facilitated by the preference of both enzymes for decay intermediates whose 5' end is monophosphorylated. In this report we identify key characteristics of RNA that influence the rate of 5'-monophosphate-assisted cleavage by these two ribonucleases. In vitro, both require at least two and prefer three or more unpaired 5'-terminal nucleotides for such cleavage; however, RNase G is impeded more than RNase E when fewer than four unpaired nucleotides are present at the 5' end. Each can tolerate any unpaired nucleotide (A, G, C, or U) at either of the first two positions, with only modest biases. The optimal spacing between the 5' end and the scissile phosphate appears to be eight nucleotides for RNase E but only six for RNase G. 5'-Monophosphate-assisted cleavage also occurs, albeit more slowly, when that spacing is greater or at most one nucleotide shorter than the optimum, but there is no simple inverse relationship between increased spacing and the rate of cleavage. These properties are also manifested during 5'-end-dependent mRNA degradation in E. coli.