Quality control of ribosomal RNA mediated by polynucleotide phosphorylase and RNase R.
Despite their overall accuracy, errors in macromolecular processes, such as rRNA synthesis and ribosome assembly, inevitably occur. However, whether these errors are remediated and how this might be accomplished is not known. In previous work, we showed that a double mutant strain lacking both polynucleotide phosphorylase (PNPase) and RNase R activities is inviable. In the course of examining the molecular basis for this phenotype, we found that shifting a temperature-sensitive mutant strain to 42 degrees C led to cessation of growth and loss of cell viability. Northern analysis of RNA isolated from such cells after the temperature shift revealed that fragments of 16S and 23S rRNA accumulated to a high level, and that the amount of ribosomes and ribosomal subunits decreased due to defects in ribosome assembly. rRNA fragments were not detected at 31 degrees C or when single mutant strains were grown at 42 degrees C. Pulse-chase analysis showed that the rRNA fragments appeared within 5 min at 42 degrees C, and that they accumulated before the loss of cell viability. The data are consistent with a model in which PNPase and RNase R mediate a previously unknown quality control process that normally removes defective rRNAs as soon as they are generated. In the absence of these RNases, rRNA fragments accumulate, leading to interference with ribosome maturation and ultimately to cell death.
Department of Biochemistry and Molecular Biology, University of Miami School of Medicine, FL 33101, USA.
Zhuan-Fen
ZF
Cheng
Murray P
MP
Deutscher
D001483:Base Sequence; D015152:Blotting, Northern; D017931:DNA Primers; D016521:Electrophoresis, Gel, Pulsed-Field; D004722:Endoribonucleases / Q000378:metabolism*; D005095:Exoribonucleases; D006358:Hot Temperature; D011117:Polyribonucleotide Nucleotidyltransferase / Q000378:metabolism*; D011786:Quality Control; D012335:RNA, Ribosomal / Q000378:metabolism*
D017931:DNA Primers; D012335:RNA, Ribosomal; D011117:Polyribonucleotide Nucleotidyltransferase; D004722:Endoribonucleases; D005095:Exoribonucleases; C080868:ribonuclease R