Polyribosome-Dependent Clustering of Membrane-Anchored RNA Degradosomes To Form Sites of mRNA Degradation in Escherichia coli.

The essential endoribonuclease RNase E, which is a component of the Escherichia coli multienzyme RNA degradosome, has a global role in RNA processing and degradation. RNase E localizes to the inner cytoplasmic membrane in small, short-lived clusters (puncta). Rifampin, which arrests transcription, inhibits RNase E clustering and increases its rate of diffusion. Here, we show that inhibition of clustering is due to the arrest of transcription using a rifampin-resistant control strain. Two components of the RNA degradosome, the 3' exoribonuclease polynucleotide phosphorylase (PNPase) and the DEAD box RNA helicase RhlB, colocalize with RNase E in puncta. Clustering of PNPase and RhlB is inhibited by rifampin, and their diffusion rates increase, as evidenced by photobleaching measurements. Results with rifampin treatment reported here show that RNA degradosome diffusion is constrained by interaction with RNA substrate. Kasugamycin, which arrests translation initiation, inhibits formation of puncta and increases RNA degradosome diffusion rates. Since kasugamycin treatment results in continued synthesis and turnover of ribosome-free mRNA but inhibits polyribosome formation, RNA degradosome clustering is therefore polyribosome dependent. Chloramphenicol, which arrests translation elongation, results in formation of large clusters (foci) of RNA degradosomes that are distinct from puncta. Since chloramphenicol-treated ribosomes are stable, the formation of RNA degradosome foci could be part of a stress response that protects inactive polyribosomes from degradation. Our results strongly suggest that puncta are sites where translationally active polyribosomes are captured by membrane-associated RNA degradosomes. These sites could be part of a scanning process that is an initial step in mRNA degradation. Here, we show that RNase E, RhlB, and PNPase act together as components of the multienzyme RNA degradosome in polyribosome-dependent clustering to form puncta on the inner cytoplasmic membrane. Our results support the hypothesis that RNA degradosome puncta are sites of mRNA degradation. We propose that clustering of RNA degradosomes is a pre-RNase E cleavage step in which polyribosomes are scanned in a search for ribosome-free mRNA. This work is part of an emerging view that posttranscriptional events such as tRNA maturation, late steps in ribosome assembly, and mRNA degradation are membrane associated and partitioned from translation in the cytoplasm and transcription in the nucleoid. This separation could protect newly synthesized transcripts from premature destructive interactions with the RNA degradosome. The scanning of ribosomes and polyribosomes could be part of a general mechanism in which defective stable RNA or ribosome-free mRNA is targeted for destruction by the RNA degradosome.