A new ribosomal mutation which affects the two ribosomal subunits in Escherichia coli

Ribosome biogenesis and the translation process in Escherichia coli

Translation, the decoding of mRNA into protein, is the third and final element of the central dogma. The ribosome, a nucleoprotein particle, is responsible and essential for this process. The bacterial ribosome consists of three rRNA molecules and approximately 55 proteins, components that are put together in an intricate and tightly regulated way. When finally matured, the quality of the particle, as well as the amount of active ribosomes, must be checked. The focus of this review is ribosome biogenesis in Escherichia coli and its cross-talk with the ongoing protein synthesis. We discuss how the ribosomal components are produced and how their synthesis is regulated according to growth rate and the nutritional contents of the medium. We also present the many accessory factors important for the correct assembly process, the list of which has grown substantially during the last few years, even though the precise mechanisms and roles of most of the proteins are not understood.

Thermus thermophilus L4 ribosomal protein: purification and sensitivity alteration against erythromycin of E. coli cells harboring a single amino acid mutant of TthL4 within its extended loop

Protein L4 from the thermophilic bacterium Thermus thermophilus (TthL4) was heterologously overproduced in Escherichia coli cells and purified under native conditions by using ion exchange chromatography. Although it's known strong binding to RNA (23S rRNA as well as mRNA) the yield of the purified protein was 6 mg per 10 g of cells and it is similar to that referred for Thermotoga maritima L4 ribosomal protein. In addition, E. coli cells harboring the wild type Thermus thermophilus L4 (wtTthL4) ribosomal protein as well as its mutant having changed the highly conserved glutamic acid 56 by alanine (TthL4-Ala 56) were incorporated into E. coli ribosomes after transformation of the host cells with the recombined vector. The cells having incorporated the mutant TthL4-Ala56 are more sensitive against erythromycin related to that containing the wtTthL4 protein.The resistance to the drug indicates that the mutated amino acid Glu56 is probably critical for the local ribosomal conformation and that its mutation induces conformational disturbances that are "transferred" to the entrance of the major exit tunnel, the place where the drug does bind.

Assembly of the 30S ribosomal subunit

Ribosomes are large macromolecular complexes responsible for cellular protein synthesis. The smallest known cytoplasmic ribosome is found in prokaryotic cells; these ribosomes are about 2.5 MDa and contain more than 4000 nucleotides of RNA and greater than 50 proteins. These components are distributed into two asymmetric subunits. Recent advances in structural studies of ribosomes and ribosomal subunits have revealed intimate details of the interactions within fully assembled particles. In contrast, many details of how these massive ribonucleoprotein complexes assemble remain elusive. The goal of this review is to discuss some crucial aspects of 30S ribosomal subunit assembly.

The polypeptide tunnel system in the ribosome and its gating in erythromycin resistance mutants of L4 and L22

Variations in the inner ribosomal landscape determining the topology of nascent protein transport have been studied by three-dimensional cryo-electron microscopy of erythromycin-resistant Escherichia coli 70S ribosomes. Significant differences in the mouth of the 50S subunit tunnel system visualized in the present study support a simple steric-hindrance explanation for the action of the drug. Examination of ribosomes in different functional states suggests that opening and closing of the main tunnel are dynamic features of the large subunit, possibly accompanied by changes in the L7/L12 stalk region. The existence and dynamic behavior of side tunnels suggest that ribosomal proteins L4 and L22 might be involved in the regulation of a multiple exit system facilitating cotranslational processing (or folding or directing) of nascent proteins.

Erythromycin inhibits the assembly of the large ribosomal subunit in growing Escherichia coli cells

Erythromycin and other macrolide antibiotics have been examined for their effects on ribosome assembly in growing Escherichia coli cells. Formation of the 50S ribosomal subunit was specifically inhibited by erythromycin and azithromycin. Other related compounds tested, including oleandomycin, clarithromycin, spiramycin, and virginiamycin M1, did not influence assembly. Erythromycin did not promote the breakdown of ribosomes formed in the absence of the drug. Two erythromycin-resistant mutants with alterations in ribosomal proteins L4 and L22 were also examined for an effect on assembly. Subunit assembly was affected in the mutant containing the L22 alteration only at erythromycin concentrations fourfold greater than those needed to stop assembly in wild-type cells. Ribosomal subunit assembly was only marginally affected at the highest drug concentration tested in the cells that contained the altered L4 protein. These novel results indicate that erythromycin has two effects on translation, preventing elongation of the polypeptide chain and also inhibiting the formation of the large ribosomal subunit.

Ribosomal protein gene sequence changes in erythromycin-resistant mutants of Escherichia coli

The genes for ribosomal proteins L4 and L22 from two erythromycin-resistant mutants of Escherichia coli have been isolated and sequenced. In the L4 mutant, an A-to-G transition in codon 63 predicted a Lys-to-Glu change in the protein. In the L22 strain, a 9-bp deletion removed codons 82 to 84, eliminating the sequence Met-Lys-Arg from the protein. Consistent with these DNA changes, in comparison with wild-type proteins, both mutant proteins had reduced first-dimension mobilities in two-dimensional polyacrylamide gels. Complementation of each mutation by a wild-type gene on a plasmid vector resulted in increased erythromycin sensitivity in the partial-diploid strains. The fraction of ribosomes containing the mutant form of the protein was increased by growth in the presence of erythromycin. Erythromycin binding was increased by the fraction of wild-type protein present in the ribosome population. The strain with the L4 mutation was found to be cold sensitive for growth at 20 degrees C, and 50S-subunit assembly was impaired at this temperature. The mutated sequences are highly conserved in the corresponding proteins from a number of species. The results indicate the participation of these proteins in the interaction of erythromycin with the ribosome.

Elongating ribosomes in vivo are refractory to erythromycin

We have studied the kinetics of erythromycin inhibition of translation in growing bacteria. In order to simplify the interpretation of our data, we have used a mutant (envA), known to have an increased permeability to several antibiotics, including erythromycin. The data clearly show that an initial stage of translation is sensitive to erythromycin, but that the elongating ribosome is insensitive to the antibiotic.

Erythromycin resistance due to a mutation in a ribosomal RNA operon of Escherichia coli

There are seven ribosomal RNA operons (rrn operons) in Escherichia coli. A single rrn operon was amplified by use of a multicopy recombinant plasmid containing a complete rrnH operon. rrnH thereby has the potential to contribute a greater fraction of the rRNA found in ribosomes. Erythromycin-resistant mutants were isolated from cells containing the plasmid, and at least one mutation to resistance was shown to reside in rrnH on the plasmid. Erythromycin resistance was retained when a major deletion was introduced into the 16S rRNA gene and was abolished by deletions that affect the 16S and 23S rRNA genes but do not alter the 5S rRNA gene or non-rrnH DNA. Cell-free S30 protein-synthesizing extracts from cells containing the mutant plasmid have an increased resistance to erythromycin. The selection procedure used to isolate erythromycin-resistance mutations in rrnH may allow, with minor modifications, the isolation of mutations in rrn operons that change resistance of the ribosome to other antibiotics or that alter other properties of ribosomes.

Properties of ribosomes from erythromycin resistant mutants of Escherichia coli

We have studied the in vitro properties of ribosomes from several mutants resistant to erythromycin. Mutations in three different genes may confer resistance to erythromycin. Two of them are structural genes for proteins L4 and L22 of the large subunit. The third mutation (in eryC gene) seems to affect mainly the small subunit. The mechanism of action of the antibiotic may involve both subunits.