Authentic precursors to ribosomal subunits accumulate in Escherichia coli in the absence of functional DnaK chaperone.
Escherichia coli dnaK-ts mutants are defective in the late stages of ribosome biogenesis at high temperature. Here, we show that the 21S, 32S and 45S ribosomal particles that accumulate in the dnaK756-ts mutant at 44 degrees C contain unprocessed forms of their 16S and 23S rRNAs (partially processed in the case of 45S particles). Their 5S rRNA stoichiometry and ribosomal protein composition are typical of the genuine ribosomal precursors found in a wild-type (dnaK+) strain. Despite the lack of a functional DnaK, a very slow maturation of these 21S, 32S and 45S particles to structurally and functionally normal 30S and 50S ribosomal subunits still occurs at high temperature. This conversion is accompanied by the processing of p16S and p23S rRNAs to their mature forms. We conclude that: (i) 21S, 32S and 45S particles are not dead-end particles, but true precursors to active ribosomes (21S particles are converted to 30S subunits, and 32S and 45S to 50S subunits); (ii) DnaK is not absolutely necessary for ribosome biogenesis, but accelerates the late steps of this process considerably at high temperature; and (iii) 23S rRNA processing depends on the stage reached in the stepwise assembly of the 50S subunit, not directly on DnaK.
El Hage
Aziz
A
Institut de Biologie Physico-Chimique, UPR 9073 du CNRS, and University Paris 7--Denis Diderot, 13 rue Pierre et Marie Curie, 75005 Paris, France.
Alix
Jean-Hervé
JH
D001483:Base Sequence; D004269:DNA, Bacterial / Q000235:genetics; D004275:DNA, Ribosomal / Q000235:genetics; D004926:Escherichia coli / Q000187:drug effects / Q000235:genetics* / Q000648:ultrastructure; D029968:Escherichia coli Proteins / Q000235:genetics*; D017353:Gene Deletion; D018840:HSP70 Heat-Shock Proteins / Q000235:genetics*; D007700:Kinetics; D018832:Molecular Chaperones / Q000235:genetics*; D015345:Oligonucleotide Probes; D012329:RNA, Bacterial / Q000235:genetics; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D012338:RNA, Ribosomal, 23S / Q000235:genetics; D012341:RNA, Ribosomal, 5S / Q000235:genetics; D012270:Ribosomes / Q000235:genetics / Q000648:ultrastructure*; D012293:Rifampin / Q000494:pharmacology
D004269:DNA, Bacterial; D004275:DNA, Ribosomal; D029968:Escherichia coli Proteins; D018840:HSP70 Heat-Shock Proteins; D018832:Molecular Chaperones; D015345:Oligonucleotide Probes; D012329:RNA, Bacterial; D012336:RNA, Ribosomal, 16S; D012338:RNA, Ribosomal, 23S; D012341:RNA, Ribosomal, 5S; C063842:dnaK protein, E coli; D012293:Rifampin