Abstract
Decay of pre-existing ribonucleic acid was studied in Escherichia coli cells subjected to high temperature or to starvation for nitrogen, phosphate, amino acids, or a carbon source. In these studies a series of mutants affected in ribonucleic I(RNase I, EC 3.1.4.22) polynucleotide phosphorylase (EC 2.7.7.8) or ribonuclease II (RNase II, EC 3.1.4.23) were used. Degradation of total RNA and the disappearance of 23 S and 16 S rRNA were followed. The results obtained indicated that, by and large, decay of 23 S and 16 S RNA parallels that of total RNA. Decay of RNA depended on the nuclease content of the cells as well as on the treatment of applied. It was most pronounced during carbon starvation and least in cells deprived of phosphate ions. It was most effective in strains containing all three nucleases and least in the strain defective in all three. The exonucleases polynucleotide phosphorylase and RNase II did not seem to affect the extent of 23 S and 16 S RNA disappearance. Strains with modified exonucleases did accumulate low molecular weight RNA species during treatments which induced considerable degradation of 23 S and 16 S RNA. Based on the above date and previous observations, we suggest that during various starvations a similar mechanism is operative. The 23 S and 16 S RNAs are degraded endonucleolytically, and this is the rate-limiting step during starvation. The exonucleases polynucleotide phosphorylase and RNase II seem to participate primarily in the decay of the low molecular weight RNA species formed by the endonuclease(s), not as yet identified.
Highlights
Degradation of total RNA and the disappearance of 23 S and 16 S rRNA were followed
We had earlier studied the process of ribosome decay in E. coli subjected to carbon starvation [5, 6] and suggested that endonucleolytic degradation of rRNA is the first step in the degradative process; after the endonucleolytic attack, which probably occurs in the ribosomal subunits [6], the particle falls apart and the RNA pieces generated by the endonuclease are degraded to acid-soluble material by the exonucleases RNase II and polynucleotide phosphorylase
The strains used in this study were AB301 (RNase I+: PNPase+, RNase II+), PRlOO (RNase I, PNPase+, RNase II+), PR13 (RNase I, PNPasemod, RNase II+), and N7060 (RNase I, PNPasemod, RNase IImOd). (Strains PRlOO and PR13 are isogenic.) Previous studies on RNA degradation during carbon starvation [5, 6] have shown that rRNA degradation is qualitatively similar at 45” and at 50”
Summary
$ Permanent address, Department versity, Tel-Aviv, Israel. A single mechanism in which endonucleolytic cleavages are followed by exonucleolytic attacks could explain the decay of ribosomes during most starvations
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