Intermediates in the assembly of ribosomes by a mutant of Escherichia coli

Inhibition of bacterial ribosome assembly: a suitable drug target?

The assembly of bacterial ribosomes is viewed with increasing interest as a potential target for new antibiotics. The in vivo synthesis and assembly of ribosomes are briefly reviewed here, highlighting the many ways in which assembly can be perturbed. The process is compared with the model in vitro process from which much of our knowledge is derived. The coordinate synthesis of the ribosomal components is essential for their ordered and efficient assembly; antibiotics interfere with this coordination and therefore affect assembly. It has also been claimed that the binding of antibiotics to nascent ribosomes prevents their assembly. These two contrasting models of antibiotic action are compared and evaluated. Finally, the suitability and tractability of assembly as a drug target are assessed.

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.

The DEAD-box RNA helicase SrmB is involved in the assembly of 50S ribosomal subunits in Escherichia coli

Ribosome assembly in Escherichia coli involves 54 ribosomal proteins and three RNAs. Whereas functional subunits can be reconstituted in vitro from the isolated components, this process requires long incubation times and high temperatures compared with the in vivo situation, suggesting that non-ribosomal factors facilitate assembly in vivo. Here, we show that SrmB, a putative DEAD-box RNA helicase, is involved in ribosome assembly. The deletion of the srmB gene causes a slow-growth phenotype at low temperature. Polysome profile analyses of the corresponding cells reveal a deficit in free 50S ribosomal subunits and the accumulation of a new particle sedimenting around 40S. Analysis of the ribosomal RNA and protein contents of the 40S particle indicates that it represents a large subunit that is incompletely assembled. In particular, it lacks L13, one of the five ribosomal proteins that are essential for the early assembly step in vitro. Sucrose gradient fractionation also shows that, in wild-type cells, SrmB associates with a pre50S particle. From our results, we propose that SrmB is involved in an early step of 50S assembly that is necessary for the binding of L13. This step may consist of a structural rearrangement that, at low temperature, cannot occur without the assistance of this putative RNA helicase.

The effects of mutations in the rpmB,G operon of Escherichia coli on ribosome assembly and ribosomal protein synthesis

The rpmB,G operon of Escherichia coli codes for proteins L28 and L33 of the larger (50S) ribosomal subunit. Strains with mutations in this operon can help define the roles of these proteins in ribosome synthesis and function. One such strain, BM108, makes neither protein and is unable to synthesize completed ribosomes; instead ribonucleoproteins accumulate, in the form of '30S material' and '47S particles'. However, when protein L28 is supplied from a plasmid, the growth rate, the kinetics of ribosome synthesis and the coordination of ribosomal protein synthesis are no different from that in wild-type organisms even though protein L33 is missing. This suggests that the latter protein can be redundant for ribosome synthesis and function. Another mutant strain, BM81, has a frameshift mutation that gives rise to an oversized protein L28. This mutant accumulates 30S material and 47S particles during slow exponential growth. The composition of the 47S particles from strains BM81, BM108 and a third mutant strain, TP28, suggests that their defining feature is the absence of L28; this is further evidence for an important role for this protein in ribosome assembly. Accumulation of ribonucleoproteins in strains BM81 and BM108 leads to some loss of the ordinarily precise coordination of synthesis of ribosomal proteins. We describe and discuss the characteristic features of this unbalanced synthesis.

Ribosomal protein synthesis by a mutant of Escherichia coli

The mutant strain of Escherichia coli, TP28, synthesises ribosomes by an abnormal pathway and accumulates large quantities of 47S ribonucleoprotein particles. The protein complement of mutant 70S ribosomes is normal but 47S particles contain only traces of proteins L28 and L33 and have a significantly reduced content of four other proteins. The mutation reduces the rates of synthesis of L28 and L33 by about half but other widespread alterations ensue. In particular, ribosomal protein synthesis in the mutant strain becomes less well balanced than in its parent: some proteins, particularly those from promoter-proximal genes, are oversynthesized and their excess then degraded.

Defective processing of ribosomal precursor RNA in Saccharomyces cerevisiae

Saccharomyces cerevisiae (strain A224A) has an abnormal distribution of cytoplasmic ribosomal subunits when grown at 36 degrees C, with sucrose-gradient analysis of extracts revealing an apparent excess of material sedimenting at 60 S. This abnormality is not observed at either 23 degrees C or 30 degrees C. At 36 degrees C the defect(s) is expressed as a slowed conversion of 20 S ribosomal precursor RNA to mature 18 S rRNA, although the corresponding maturation of 27 S ribosomal precursor RNA to mature 25 S rRNA is normal. Studies on this yeast strain and on mutants derived from it may help to elucidate the role(s) of individual ribosomal components in controlling ribosome biogenesis in eukaryotes.