Influence of translation on RppH-dependent mRNA degradation in Escherichia coli.

In Escherichia coli, the endonuclease RNase E can access internal cleavage sites in mRNA either directly or by a 5' end-dependent mechanism in which cleavage is facilitated by prior RppH-catalysed conversion of the 5'-terminal triphosphate to a monophosphate, to which RNase E can bind. The characteristics of transcripts that determine which of these two pathways is primarily responsible for their decay are poorly understood. Here we report the influence of ribosome binding and translocation on each pathway, using yeiP and trxB as model transcripts. Ribosome binding to the translation initiation site impedes degradation by both mechanisms. However, because the effect on the rate of 5' end-independent decay is greater, poor ribosome binding favours degradation by that pathway. Arresting translation elongation with chloramphenicol quickly inhibits RNase E cleavage downstream of the initiation codon but has little or no immediate effect on cleavage upstream of the ribosome binding site. RNase E binding to a monophosphorylated 5' end appears to increase the likelihood of cleavage at sites within the 5' untranslated region. These findings indicate that ribosome binding and translocation can have a major impact on 5' end-dependent mRNA degradation in E. coli and suggest a possible sequence of events that follow pyrophosphate removal.