Degradation of Ribosomes in Escherichia coli

The number of ribosomes present in cells changes as physiological conditions warrant. For example, when growing E. coli cells are transferred to a starvation medium, ribosome degradation ensues. Although this degradation is well known, the enzymes responsible have not been elucidated. In order to identify RNases involved in ribosome degradation, we have developed two in vitro assays that measure this process. The first allows visualization of the products of ribosome degradation on a denaturing polyacrylamide gel. In the second, degradation is assessed by measurement of acid‐soluble radioactivity released from 32 P labeled ribosomes. Together these two assays have provided evidence for a multi‐step degradation process involving both endo‐ and exoribonucleases. These data support a model originally proposed by Kaplan and Apirion (1975) in which an endoribonuclease initially cleaves the rRNA and the resulting fragments are further degraded by exoribonucleases. Examination of extracts from strains deficient in known RNases revealed that the endoribonucleases, RNase E and RNase G, are involved in the initial cleavages. The resulting fragments are degraded to mononucleotides primarily by the 3′‐5′ exoribonucleases, RNase R and polynucleotide phosphorylase, with perhaps some contribution by RNase II as well. Studies are in progress to assess the role of these RNases in ribosome degradation in vivo .