The site of ribosome degradation in starved Escherichia coli cells.

An attempt was made, in starved Escherichia coli cells, to locate the site at which the process of ribosome degradation is initiated. Supernatant and rapidly sedimenting pellet fractions from exponentially growing and from carbon-starved cells were prepared, and the ribonucleic acids from these fractions were seperated by polyacrylamide electrophoresis and quantitated. The data indicated that 23 S, 16 S, and 5 S RNA are lost only from the pellet; and also that the low molecular weight RNA degradation products are confined to this fraction. Ribosomes from supernatant and pellet fractions were seperated on sucrose density gradients. The sedimentation profiles obtained indicated that pellet fractions of starved cells contain for the most part 50 S and 30 S subunits, whereas 70 S monosomes were most abundant in the supernatant fraction. In vitro measurements on RNA degradation in supernatant and rapidly sedimenting pellet fractions confirmed the in vivo data on the exclusive degradation of ribosomal RNA in the pellet. Based on these data and on previous observations, we suggest that the endoribonucleolytic attack which triggers ribosome degradation, occurs in free subunits attached to the rapidly sedimenting membrane fraction. Subsequently, the ribosome falls apart, and the small RNA pieces generated remain attached to the pellet fraction until their final degradation by the exonuclease.