Mutagenesis of ribosomal protein S8 from Escherichia coli: defects in regulation of the spc operon

Synonymous rpsH variants: the common denominator in Escherichia coli adapting to ionizing radiation

Ionizing radiation (IR) in high doses is generally lethal to most organisms. Investigating mechanisms of radiation resistance is crucial for gaining insights into the underlying cellular responses and understanding the damaging effects of IR. In this study, we conducted a comprehensive analysis of sequencing data from an evolutionary experiment aimed at understanding the genetic adaptations to ionizing radiation in Escherichia coli. By including previously neglected synonymous mutations, we identified the rpsH c.294T > G variant, which emerged in all 17 examined isolates across four subpopulations. The identified variant is a synonymous mutation affecting the 30S ribosomal protein S8, and consistently exhibited high detection and low allele frequencies in all subpopulations. This variant, along with two additional rpsH variants, potentially influences translational control of the ribosomal spc operon. The early emergence and stability of these variants suggest their role in adapting to environmental stress, possibly contributing to radiation resistance. Our findings shed light on the dynamics of ribosomal variants during the evolutionary process and their potential role in stress adaptation, providing valuable implications for understanding clinical radiation sensitivity and improving radiotherapy.

Genome-Wide Essentiality Analysis of Mycobacterium abscessus by Saturated Transposon Mutagenesis and Deep Sequencing

Mycobacterium abscessus is an emerging opportunistic human pathogen that naturally resists most major classes of antibiotics, making infections difficult to treat. Thus far, little is known about M. abscessus physiology, pathogenesis, and drug resistance. Genome-wide analyses have comprehensively catalogued genes with essential functions in Mycobacterium tuberculosis and Mycobacterium avium subsp. hominissuis (here, M. avium) but not in M. abscessus. By optimizing transduction conditions, we achieved full saturation of TA insertion sites with Himar1 transposon mutagenesis in the M. abscessus ATCC 19977^T genome, as confirmed by deep sequencing prior to essentiality analyses of annotated genes and other genomic features. The overall densities of inserted TA sites (85.7%), unoccupied TA sites (14.3%), and nonpermissive TA sites (8.1%) were similar to results in M. tuberculosis and M. avium. Of the 4,920 annotated genes, 326 were identified as essential, 269 (83%) of which have mutual homology with essential M. tuberculosis genes, while 39 (12%) are homologous to genes that are not essential in M. tuberculosis and M. avium, and 11 (3.4%) only have homologs in M. avium. Interestingly, 7 (2.1%) essential M. abscessus genes have no homologs in either M. tuberculosis or M. avium, two of which were found in phage-like elements. Most essential genes are involved in DNA replication, RNA transcription and translation, and posttranslational events to synthesize important macromolecules. Some essential genes may be involved in M. abscessus pathogenesis and antibiotics response, including certain essential tRNAs and new short open reading frames. Our findings will help to pave the way for better understanding of M. abscessus and benefit development of novel bactericidal drugs against M. abscessus. IMPORTANCE Limited knowledge regarding Mycobacterium abscessus pathogenesis and intrinsic resistance to most classes of antibiotics is a major obstacle to developing more effective strategies to prevent and mitigate disease. Using optimized procedures for Himar1 transposon mutagenesis and deep sequencing, we performed a comprehensive analysis to identify M. abscessus genetic elements essential for in vitro growth and compare them to similar data sets for M. tuberculosis and M. avium subsp. hominissuis. Most essential M. abscessus genes have mutual homology with essential M. tuberculosis genes, providing a foundation for leveraging available knowledge from M. tuberculosis to develop more effective drugs and other interventions against M. abscessus. A small number of essential genes unique to M. abscessus deserve further attention to gain insights into what makes M. abscessus different from other mycobacteria. The essential genes and other genomic features such as short open reading frames and noncoding RNA identified here will provide useful information for future study of M. abscessus pathogenicity and new drug development.

Structure analysis of free and bound states of an RNA aptamer against ribosomal protein S8 from Bacillus anthracis

Several protein-targeted RNA aptamers have been identified for a variety of applications and although the affinities of numerous protein-aptamer complexes have been determined, the structural details of these complexes have not been widely explored. We examined the structural accommodation of an RNA aptamer that binds bacterial r-protein S8. The core of the primary binding site for S8 on helix 21 of 16S rRNA contains a pair of conserved base triples that mold the sugar-phosphate backbone to S8. The aptamer, which does not contain the conserved sequence motif, is specific for the rRNA binding site of S8. The protein-free RNA aptamer adopts a helical structure with multiple non-canonical base pairs. Surprisingly, binding of S8 leads to a dramatic change in the RNA conformation that restores the signature S8 recognition fold through a novel combination of nucleobase interactions. Nucleotides within the non-canonical core rearrange to create a G-(G-C) triple and a U-(A-U)-U quartet. Although native-like S8-RNA interactions are present in the aptamer-S8 complex, the topology of the aptamer RNA differs from that of the helix 21-S8 complex. This is the first example of an RNA aptamer that adopts substantially different secondary structures in the free and protein-bound states and highlights the remarkable plasticity of RNA secondary structure.

Bacterial RNA motif in the 5' UTR of rpsF interacts with an S6:S18 complex

Approximately half the transcripts encoding ribosomal proteins in Escherichia coli include a structured RNA motif that interacts with a specific ribosomal protein to inhibit gene expression, thus allowing stoichiometric production of ribosome components. However, many of these RNA structures are not widely distributed across bacterial phyla. It is increasingly common for RNA motifs associated with ribosomal protein genes to be identified using comparative genomic methods, yet these are rarely experimentally validated. In this work, we characterize one such motif that precedes operons containing rpsF and rpsR, which encode ribosomal proteins S6 and S18. This RNA structure is widely distributed across many phyla of bacteria despite differences within the downstream operon, and examples are present in both E. coli and Bacillus subtilis. We demonstrate a direct interaction between an example of the RNA from B. subtilis and an S6:S18 complex using in vitro binding assays, verify our predicted secondary structure, and identify a putative protein-binding site. The proposed binding site bears a strong resemblance to the S18 binding site within the 16S rRNA, suggesting molecular mimicry. This interaction is a valuable addition to the canon of ribosomal protein mRNA interactions. This work shows how experimental verification translates computational results into concrete knowledge of biological systems.

Proteomic characterization of evolutionarily conserved and variable proteins of Arabidopsis cytosolic ribosomes

Analysis of 80S ribosomes of Arabidopsis (Arabidopsis thaliana) by use of high-speed centrifugation, sucrose gradient fractionation, one- and two-dimensional gel electrophoresis, liquid chromatography purification, and mass spectrometry (matrix-assisted laser desorption/ionization time-of-flight and electrospray ionization) identified 74 ribosomal proteins (r-proteins), of which 73 are orthologs of rat r-proteins and one is the plant-specific r-protein P3. Thirty small (40S) subunit and 44 large (60S) subunit r-proteins were confirmed. In addition, an ortholog of the mammalian receptor for activated protein kinase C, a tryptophan-aspartic acid-domain repeat protein, was found to be associated with the 40S subunit and polysomes. Based on the prediction that each r-protein is present in a single copy, the mass of the Arabidopsis 80S ribosome was estimated as 3.2 MD (1,159 kD 40S; 2,010 kD 60S), with the 4 single-copy rRNAs (18S, 26S, 5.8S, and 5S) contributing 53% of the mass. Despite strong evolutionary conservation in r-protein composition among eukaryotes, Arabidopsis 80S ribosomes are variable in composition due to distinctions in mass or charge of approximately 25% of the r-proteins. This is a consequence of amino acid sequence divergence within r-protein gene families and posttranslational modification of individual r-proteins (e.g. amino-terminal acetylation, phosphorylation). For example, distinct types of r-proteins S15a and P2 accumulate in ribosomes due to evolutionarily divergence of r-protein genes. Ribosome variation is also due to amino acid sequence divergence and differential phosphorylation of the carboxy terminus of r-protein S6. The role of ribosome heterogeneity in differential mRNA translation is discussed.

Contributions of pseudoknots and protein SmpB to the structure and function of tmRNA in trans-translation

Bacteria contain transfer-messenger RNA (tmRNA), a molecule that during trans-translation tags incompletely translated proteins with a small peptide to signal the proteolytic destruction of defective polypeptides. TmRNA is composed of tRNA- and mRNA-like domains connected by several pseudoknots. Using truncated ribosomal protein L27 as a reporter for tagging in vitro and in vivo, we have developed exceptionally sensitive assays to study the role of Escherichia coli tmRNA in trans-translation. Site-directed mutagenesis experiments showed that pseudoknot 2 and the abutting helix 5 were particularly important for the binding of ribosomal protein S1 to tmRNA. Pseudoknot 4 not only facilitated tmRNA maturation but also promoted tagging. In addition, the three pseudoknots (pk2 to pk4) were shown to play a significant role in the proper folding of the tRNA-like domain. Protein SmpB enhanced tmRNA processing, suggesting a new role for SmpB in trans-translation. Taken together, these results provide unanticipated insights into the functions of the pseudoknots and protein SmpB during tmRNA folding, maturation, and protein synthesis.

The structure of a ribosomal protein S8/spc operon mRNA complex

In bacteria, translation of all the ribosomal protein cistrons in the spc operon mRNA is repressed by the binding of the product of one of them, S8, to an internal sequence at the 5' end of the L5 cistron. The way in which the first two genes of the spc operon are regulated, retroregulation, is mechanistically distinct from translational repression by S8 of the genes from L5 onward. A 2.8 A resolution crystal structure has been obtained of Escherichia coli S8 bound to this site. Despite sequence differences, the structure of this complex is almost identical to that of the S8/helix 21 complex seen in the small ribosomal subunit, consistent with the hypothesis that autogenous regulation of ribosomal protein synthesis results from conformational similarities between mRNAs and rRNAs. S8 binding must repress the translation of its own mRNA by inhibiting the formation of a ribosomal initiation complex at the start of the L5 cistron.

Translational feedback regulation of the gene for L35 in Escherichia coli requires binding of ribosomal protein L20 to two sites in its leader mRNA: a possible case of ribosomal RNA-messenger RNA molecular mimicry

In addition to being a component of the large ribosomal subunit, ribosomal protein L20 of Escherichia coli also acts as a translational repressor. L20 is synthesized from the IF3 operon that contains three cistrons coding for IF3, and ribosomal proteins L35 and L20. L20 directly represses the expression of the gene encoding L35 and the expression of its own gene by translational coupling. All of the cis-acting sequences required for repression by L20, called the operator, are found on an mRNA segment extending from the middle of the IF3 gene to the start of the L35 gene. L20-mediated repression requires a long-range base-pairing interaction between nucleotide residues within the IF3 gene and residues just upstream of the L35 gene. This interaction results in the formation of a pseudoknot. Here we show that L20 causes protection of nucleotide residues in two regions of the operator in vitro. The first region is the pseudoknot itself and the second lies in an irregular stem located upstream of the L35 gene. By primer extension analysis, we show that L20 specifically induces reverse transcriptase stops in both regions. Therefore, these two regions define two L20-binding sites in the operator. Using mutations and deletions of rpml'-'lacZ fusions, we show that both sites are essential for repression in vivo. However L20 can bind to each site independently in vitro. One site is similar to the L20-binding site on 23S rRNA. Here we propose that L20 recognizes its mRNA and its rRNA in similar way.

Genes for two mitochondrial ribosomal proteins in flowering plants are derived from their chloroplast or cytosolic counterparts

Often during flowering plant evolution, ribosomal protein genes have been lost from the mitochondrion and transferred to the nucleus. Here, we show that substitution by a duplicated, divergent gene originally encoding the chloroplast or cytosolic ribosomal protein counterpart accounts for two missing mitochondrial genes in diverse angiosperms. The rps13 gene is missing from the mitochondrial genome of many rosids, and a transferred copy of this gene is not evident in the nucleus of Arabidopsis, soybean, or cotton. Instead, these rosids contain a divergent nuclear copy of an rps13 gene of chloroplast origin. The product of this gene from all three rosids was shown to be imported into isolated mitochondria but not into chloroplasts. The rps8 gene is missing from the mitochondrion and nucleus of all angiosperms examined. A divergent copy of the gene encoding its cytosolic counterpart (rps15A) was identified in the nucleus of four angiosperms and one gymnosperm. The product of this gene from Arabidopsis and tomato was imported successfully into mitochondria. We infer that rps13 was lost from the mitochondrial genome and substituted by a duplicated nuclear gene of chloroplast origin early in rosid evolution, whereas rps8 loss and substitution by a gene of nuclear/cytosolic origin occurred much earlier, in a common ancestor of angiosperms and gymnosperms.

Tagging ribosomal protein S7 allows rapid identification of mutants defective in assembly and function of 30 S subunits

Ribosomal protein S7 nucleates folding of the 16 S rRNA 3' major domain, which ultimately forms the head of the 30 S ribosomal subunit. Recent crystal structures indicate that S7 lies on the interface side of the 30 S subunit, near the tRNA binding sites of the ribosome. To map the functional surface of S7, we have tagged the protein with a Protein Kinase A recognition site and engineered alanine substitutions that target each exposed, conserved residue. We have also deleted conserved features of S7, using its structure to guide our design. By radiolabeling the tag sequence using Protein Kinase A, we are able to track the partitioning of each mutant protein into 30 S, 70 S, and polyribosome fractions in vivo. Overexpression of S7 confers a growth defect, and we observe a striking correlation between this phenotype and proficiency in 30 S subunit assembly among our collection of mutants. We find that the side chain of K35 is required for efficient assembly of S7 into 30 S subunits in vivo, whereas those of at least 17 other conserved exposed residues are not required. In addition, an S7 derivative lacking the N-terminal 17 residues causes ribosomes to accumulate on mRNA to abnormally high levels, indicating that our approach can yield interesting mutant ribosomes.

The location of protein S8 and surrounding elements of 16S rRNA in the 70S ribosome from combined use of directed hydroxyl radical probing and X-ray crystallography

Ribosomal protein S8, which is essential for the assembly of the central domain of 16S rRNA, is one of the most thoroughly studied RNA-binding proteins. To map its surrounding RNA in the ribosome, we carried out directed hydroxyl radical probing of 16S rRNA using Fe(II) tethered to nine different positions on the surface of protein S8 in 70S ribosomes. Hydroxyl radical-induced cleavage was observed near the classical S8-binding site in the 620 stem, and flanking the other S8-footprinted regions of the central domain at the three-helix junction near position 650 and the 825 and 860 stems. In addition, cleavage near the 5' terminus of 16S rRNA, in the 300 region of its 5' domain, and in the 1070 region of its 3'-major domain provide information about the proximity to S8 of RNA elements not directly involved in its binding. These data, along with previous footprinting and crosslinking results, allowed positioning of protein S8 and its surrounding RNA elements in a 7.8-A map of the Thermus thermophilus 70S ribosome. The resulting model is in close agreement with the extensive body of data from previous studies using protein-protein and protein-RNA crosslinking, chemical and enzymatic footprinting, and genetics.

A second-site mutation at glutamate-257 that restores the function of the mutant yeast ribosomal protein L5 containing lysine-270,271-->arginine

A genetic approach was used to identify interacting regions of yeast ribosomal protein L5 (also known as L1, L1a, or YL3). Previous studies from our laboratory showed that residues K270 and K271 in protein L5 are essential for its function. The mutant L5 protein in which both residues were replaced by arginine residues (K270,271R) exhibited about 80% RNA binding capability compared to the wild-type and the mutant protein was assembled into the 60S ribosomal subunits in vivo. The yeast strain expressing this mutant protein in a homozygous form was lethal (Biochim. Biophys. Acta 1308 (1996) 133-141). In the present study, this non-functional mutant was used to select intragenic suppressors. A spontaneous, intragenic suppressor which contained an E257K substitution (in addition to the primary mutations) was identified. The suppressor protein bound about 60% of yeast 5S rRNA in vitro compared to the wild-type. To gain more insight into the nature of the intragenic suppressor, additional mutant proteins in which E257 was substituted by a variety of amino acids were produced by site-directed mutagenesis. The ability of each mutant protein to bind yeast 5S rRNA in vitro and to suppress the lethal effect of the double K270,271 mutation in vivo were examined. Results suggest communication between two non-contiguous domains on protein L5 and that several factors, such as electrostatic interaction and hydrogen bonding are likely to play a role in this global communication. Mutation studies on E257 alone also reveal that substitutions of this residue in L5 protein could affect cell growth under specified conditions, but a variety of changes could be tolerated without serious deleterious effects. We propose a working model in which E257 is located in a loop and the dynamic as well as the flexibility of this loop is important for L5 function.

NMR structure determination of the binding site for ribosomal protein S8 from Escherichia coli 16 S rRNA

Many cellular processes involve the preferential interaction of an RNA molecule with a specific protein. A detailed analysis of the individual protein and RNA components of these interactions can provide unique insights into the structural features important for protein-RNA recognition. Ribosomal protein S8 of Escherichia coli plays a key role in 30 S ribosomal subunit assembly through its interaction with 16 S rRNA. The binding site for protein S8 comprises a portion of helix 21, nucleotides G588 to G604 and C634 to C651. This region forms a base-paired helix that is interrupted by a non-Watson-Crick segment composed of nine phylogenetically conserved nucleotides. We have investigated the detailed structure of the conserved segment and the interaction of this region with metal ions using NMR spectroscopy. Twenty-four of the 40 calculated structures converged to similar conformations and were grouped into two families. The main difference between the families is the orientation of the base of U641. The rms deviation between the heavy-atoms of the ten lowest-energy structures is 1.24 A. The orientations of the G597.C643 base-pair and A595.(A596.U644) base-triple within the conserved core have been defined and appear to extend the proximal segment of helix 21 into the phylogenetically conserved core. The base of A642 terminates this helix by stacking against C643 and the base of U641 forms hydrogen bonds with core nucleotides. The conserved core also contains a Mg2+-binding site that promotes stabilization of the secondary and tertiary structure elements of the core. A model for the interaction of S8 with its RNA-binding site is proposed.

Crystal structure of ribosomal protein S8 from Thermus thermophilus reveals a high degree of structural conservation of a specific RNA binding site

S8 is one of the core ribosomal proteins. It binds to 16 S RNA with high affinity and independently of other ribosomal proteins. It also acts as a translational repressor in Escherichia coli by binding to its own mRNA. The structure of Thermus thermophilus S8 has been determined by the method of multiple isomorphous replacement at 2.9 A resolution and refined to a crystallographic R-factor of 16.2% (Rfree 27.5%). The two domains of the structure have an alpha/beta fold and are connected by a long protruding loop. The two molecules in the asymmetric unit of the crystal interact through an extensive hydrophobic core and form a tightly associated dimer, while symmetry-related molecules form a joint beta-sheet of mixed type. This type of protein-protein interaction could be realized within the ribosomal assembly. A comparison of the structures of T. thermophilus and Bacillus stearothermophilus S8 shows that the interdomain loop is eight residues longer in the former and reveals high structural conservation of an extensive region, located in the C-terminal domain. From mutational studies this region was proposed earlier to be involved in specific interaction with RNA. On the basis of these data and on the comparison of the two structures of S8, it is proposed that the three-dimensional structure of specific RNA binding sites in ribosomal proteins is highly conserved among different species.

Structural evidence for specific S8-RNA and S8-protein interactions within the 30S ribosomal subunit: ribosomal protein S8 from Bacillus stearothermophilus at 1.9 A resolution

BACKGROUND

Prokaryotic ribosomal protein S8 is an important RNA-binding protein that occupies a central position within the small ribosomal subunit. It interacts extensively with 16S rRNA and is crucial for the correct folding of the central domain of the rRNA. S8 also controls the synthesis of several ribosomal proteins by binding to mRNA. It binds specifically to very similar sites in the two RNA molecules.

RESULTS

S8 is divided into two tightly associated domains and contains three regions that are proposed to interact with other ribosomal components: two potential RNA-binding sites, and a hydrophobic patch that may interact with a complementary hydrophobic region of S5. The N-terminal domain fold is found in several proteins including two that bind double-stranded DNA.

CONCLUSIONS

These multiple RNA-binding sites are consistent with the role of S8 in organizing the central domain and agree with the latest models of the 16S RNA which show that the S8 location coincides with a region of complicated nucleic-acid structure. The presence in a wide variety of proteins of a region homologous to the N-terminal domain supports the idea that ribosomal proteins must represent some of the earliest protein molecules.

Multiple regions of yeast ribosomal protein L1 are important for its interaction with 5 S rRNA and assembly into ribosomes

Yeast ribosomal protein L1 binds to 5 S rRNA and can be released from 60 S ribosomal subunits as an intact ribonucleoprotein particle. To identify residues important for binding of Saccharomyces cerevisiae rpL1 to 5 S rRNA and assembly into functional ribosomes, we have isolated mutant alleles of the yeast RPL1 gene by site-directed and random mutagenesis. The rpl1 mutants were assayed for association of rpL1 with 5 S rRNA in vivo and in vitro and assembly of rpL1 into functional 60 S ribosomal subunits. Consistent with previous data implicating the importance of the carboxyl-terminal 47 amino acids of rpL1 for binding to 5 S rRNA in vitro, we find that deletion of the carboxyl-terminal 8, 25, or 44 amino acids of rpL1 confers lethality in vivo. Missense mutations elsewhere in rpL1 also affect its function, indicating that multiple regions of rpL1 are important for its association with 5 S rRNA and assembly into ribosomes.

The ribosomal protein S8 from Thermus thermophilus VK1. Sequencing of the gene, overexpression of the protein in Escherichia coli and interaction with rRNA

The gene of the ribosomal protein S8 from Thermus thermophilus VK1 has been isolated from a genomic library by hybridization of an oligonucleotide coding for the N-terminal amino acid sequence of the protein, amplified by PCR and sequenced. Nucleotide sequence reveals an open reading frame coding for a protein of 138 amino acid residues (M(r) 15,839). The codon usage shows that 94% of the codons possess G or C in the third position, and agrees with the preferential usage of codons of high G+C content in the bacteria of the genus Thermus. The amino acid sequence of the protein shows 48% identity with the protein from Escherichia coli. Ribosomal protein S8 from T. thermophilus has been expressed in E. coli under the control of the T7 promoter and purified to homogeneity by heat treatment of the extract followed by cation-exchange chromatography. Conditions were defined in which T. thermophilus protein S8 binds specifically an homologous 16S rRNA fragment containing the putative S8 binding site with an apparent association constant of 5 x 10(7) M-1. The overexpressed protein binds the rRNA with the same affinity as that extracted from T. thermophilus, indicating that the thermophilic protein is correctly folded in E. coli. The specificity of this binding is dependent on the ionic strength. The protein S8 from T. thermophilus recognizes the E. coli rRNA binding sites as efficiently as the S8 protein from E. coli. This result agrees with sequence comparisons of the S8 binding site on the small subunit rRNA from E. coli and from T. thermophilus, showing strong similarities in the regions involved in the interaction. It suggests that the structural features responsible for the recognition are conserved in the mesophilic and thermophilic eubacteria, despite structural peculiarities in the thermophilic partners conferring thermostability.

Functions of the gene products of Escherichia coli

A list of currently identified gene products of Escherichia coli is given, together with a bibliography that provides pointers to the literature on each gene product. A scheme to categorize cellular functions is used to classify the gene products of E. coli so far identified. A count shows that the numbers of genes concerned with small-molecule metabolism are on the same order as the numbers concerned with macromolecule biosynthesis and degradation. One large category is the category of tRNAs and their synthetases. Another is the category of transport elements. The categories of cell structure and cellular processes other than metabolism are smaller. Other subjects discussed are the occurrence in the E. coli genome of redundant pairs and groups of genes of identical or closely similar function, as well as variation in the degree of density of genetic information in different parts of the genome.

Mutagenesis of ribosomal protein S8 from Escherichia coli: expression, stability, and RNA-binding properties of S8 mutants

Protein S8, a 129 amino acid component of the Escherichia coli ribosome, plays an essential role in the assembly of the 30S ribosomal subunit and in the translational regulation of the spc operon by virtue of its capacity to bind specifically to rRNA and mRNA. To study structure-function relationships within the protein, we have constructed a vector for its high-level expression in vivo and developed efficient methods for its purification. Under our conditions, S8 accumulates to a level of 35% of the cellular protein and can be prepared at a purity of over 98% using either HPLC or a combination of ion-exchange and gel-filtration chromatography. The unique cysteine residue at position 126 was replaced by alanine or serine by oligonucleotide-directed mutagenesis, and the two mutant proteins, CA126 and CS126, were expressed and isolated. The effects of the mutations on the RNA-binding ability, secondary structure, and stability of S8 were assessed. CD spectra indicated that wild-type S8 and the two mutant proteins have very similar secondary structures at 25 degrees C. In addition, both mutants are metabolically stable in vivo as inferred from pulse-chase labeling and immunoprecipitation experiments. However, while CA126 exhibits the same affinity for RNA and the same susceptibility to urea and thermal denaturation as wild-type S8, CS126 is severely impaired in its ability to interact with RNA and displays a dramatic reduction in conformational stability. Our results suggest that Cys126 is unlikely to play a specific role in RNA recognition but that it is an integral part of the RNA-binding domain of protein S8.

Erythromycin and 5S rRNA binding properties of the spinach chloroplast ribosomal protein CL22

The spinach chloroplast ribosomal protein (r-protein) CL22 contains a central region homologous to the Escherichia coli r-protein L22 plus long N- and C-terminal extensions. We show in this study that the CL22 combines two properties which in E. coli ribosome are split between two separate proteins. The CL22 which binds to the 5S rRNA can also be linked to an erythromycin derivative added to the 50S ribosomal subunit. This latter property is similar to that of the E. coli L22 and suggests a similar localization in the 50S subunit. We have overproduced the r-protein CL22 and deleted forms of this protein in E. coli. We show that the overproduced CL22 binds to the chloroplast 5S rRNA and that the deleted protein containing the N- and C-terminal extensions only has lost the 5S rRNA binding property. We suggest that the central homologous regions of the CL22 contains the RNA binding domain.

The complete amino acid sequence of ribosomal protein S8 from Thermus thermophilus

Protein S8 from Thermus thermophilus consists of 138 amino acids of M(r) 15,840. Its primary structure was established using peptide sequences from two different digests. Protein S8 from T. thermophilus shares a high percentage of identity with protein S8 from Thermus aquaticus. There are some consensus sequences between proteins S8 from eubacteria, archebacteria, chloroplasts, and cyanelles.