yaaJ, the tRNA-Specific Adenosine Deaminase, Is Dispensable in Bacillus subtilis.

Post-transcriptional modifications of tRNA are crucial for their core function. The inosine (I; 6-deaminated adenosine) at the first position in the anticodon of tRNAArg(ICG) modulates the decoding capability and is generally considered essential for reading CGU, CGC, and CGA codons in eubacteria. We report here that the Bacillus subtilis yaaJ gene encodes tRNA-specific adenosine deaminase and is non-essential for viability. A ß-galactosidase reporter assay revealed that the translational activity of CGN codons was not impaired in the yaaJ-deletion mutant. Furthermore, tRNAArg(CCG) responsible for decoding the CGG codon was dispensable, even in the presence or absence of yaaJ. These results strongly suggest that tRNAArg with either the anticodon ICG or ACG has an intrinsic ability to recognize all four CGN codons, providing a fundamental concept of non-canonical wobbling mediated by adenosine and inosine nucleotides in the anticodon. This is the first example of the four-way wobbling by inosine nucleotide in bacterial cells. On the other hand, the absence of inosine modification induced +1 frameshifting, especially at the CGA codon. Additionally, the yaaJ deletion affected growth and competency. Therefore, the inosine modification is beneficial for translational fidelity and proper growth-phase control, and that is why yaaJ has been actually conserved in B. subtilis.

Identification and clinical significance of somatic oncogenic mutations in epithelial ovarian cancer.

OBJECTIVE: Epithelial ovarian cancer (EOC) is a heterogeneous disease with diverse clinicopathological features and behaviors, and its heterogeneity may be concerned with the accumulation of multiple somatic oncogenic mutations. The major goals of this study are to systematically perform the comprehensive mutational profiling in EOC patients, and investigate the associations between somatic mutations and clinicopathological characteristics. METHODS: A total of 80 surgical specimens were obtained from EOC patients who had previously undergone primary debulking surgery, and genomic DNAs were extracted from fresh-frozen tissues. We investigated mutational status in hot spot regions of 50 cancer-related genes by targeted next-generation sequencing using an Ion AmpliSeq Cancer Hotspot Panel v2 Kit. RESULTS: Validated mutations were detected in 66 of the 80 tumors (82.5%). The five most frequently mutated genes were TP53 (43.8%), PIK3CA (27.5%), KRAS (23.8%), PTEN (10%) and CTNNB1 (10%). PTEN and CTNNB1 mutations were associated with younger age. PIK3CA1, KRAS and CTNNB1 mutations were observed in early-stage, whereas TP53 mutations were more common in advanced stage. Significant associations were observed between TP53 mutation and serous carcinoma, and between KRAS mutation and mucinous carcinoma. Both PIK3CA mutation and CTNNB1 mutation were also significantly associated with endometrioid and clear cell carcinoma. The patients with PIK3CA and KRAS mutations were significantly associated with favorable progression free survival (PFS). In particular, PIK3CA mutations had more significant associations with favorable PFS than PIK3CA wild-type in the endometrioid subtype (P = 0.012). Patients with mutations only in TP53 were significantly associated with worse PFS. CONCLUSION: EOCs were heterogeneous at the genomic level and harbored somatic oncogenic mutations. Our molecular profiling may have the potential for becoming a novel stratification within histological subtypes of EOC. Further studies are needed to define molecular classification for improved clinical outcomes and treatment of EOC patients in future.

Evolution of Ribosomal Protein S14 Demonstrated by the Reconstruction of Chimeric Ribosomes in Bacillus subtilis.

Ribosomal protein S14 can be classified into three types. The first, the C+ type has a Zn2+ binding motif and is ancestral. The second and third are the C- short and C- long types, neither of which contain a Zn2+ binding motif and which are ca. 90 residues and 100 residues in length, respectively. In the present study, the C+ type S14 from Bacillus subtilis ribosomes (S14BsC+) were completely replaced by the heterologous C- long type of S14 from Escherichia coli (S14Ec) or Synechococcus elongatus (S14Se). Surprisingly, S14Ec and S14Se were incorporated fully into 70S ribosomes in B. subtilis However, the growth rates as well as the sporulation efficiency of the mutants harboring heterologous S14 were significantly decreased. In these mutants, the polysome fraction was decreased and the 30S and 50S subunits accumulated unusually, indicating that cellular translational activity of these mutants was decreased. In vitro analysis showed a reduction in the translational activity of the 70S ribosome fraction purified from these mutants. The abundance of ribosomal proteins S2 and S3 in the 30S fraction in these mutants was reduced while that of S14 was not significantly decreased. It seems likely that binding of heterologous S14 changes the structure of the 30S subunit, which causes a decrease in the assembly efficiency of S2 and S3, which are located near the binding site of S14. Moreover, we found that S3 from S. elongatus cannot function in B. subtilis unless S14Se is present.IMPORTANCE S14, an essential ribosomal protein, may have evolved to adapt bacteria to zinc-limited environments by replacement of a zinc-binding motif with a zinc-independent sequence. It was expected that the bacterial ribosome would be tolerant to replacement of S14 because of the previous prediction that the spread of C- type S14 involved horizontal gene transfer. In this study, we completely replaced the C+ type of S14 in B. subtilis ribosome with the heterologous C- long type of S14 and characterized the resulting chimeric ribosomes. Our results suggest that the B. subtilis ribosome is permissive for the replacement of S14, but coevolution of S3 might be required to utilize the C- long type of S14 more effectively.

Tumor ß-catenin expression is associated with immune evasion in non-small cell lung cancer with high tumor mutation burden.

ß-catenin expression by tumor cells suppressed dendritic cell recruitment to the tumor microenvironment in a melanoma model, resulting in fewer tumor-infiltrating lymphocytes. Immunohistochemistry was used in the present study to examine the association between the expression of ß-catenin and tumor infiltrating lymphocytes and CD11c+ cells in 122 patients with non-small cell lung cancer (NSCLC), who underwent radical surgery. ß-catenin was positive in 24% of NSCLC tumors compared with 59% of squamous cell carcinomas and 11% of adenocarcinomas. There was no significant association between the expression of ß-catenin and the frequency of CD8+ cell infiltration into tumor tissues, including the stroma. Conversely, the infiltration of CD8+ cells into tumor nests was significantly lower in ß-catenin-positive cases compared with that in negative ß-catenin cases. Similarly, CD11c+ cell infiltration was significantly lower in the ß-catenin-positive group. The ß-catenin-positive group had shorter overall survival and recurrence-free survival times compared with that in the negative group. Furthermore, ß-catenin-positive NSCLC had a high tumor mutation burden, but tended to have a low expression of programmed death-ligand 1. In conclusion, the expression of ß-catenin in NSCLC was negatively associated with CD11c+ cells and cytotoxic T cell infiltration at the tumor site and had a tendency towards a poor prognosis.

Clinical relevance of oncogenic driver mutations identified in endometrial carcinoma.

PURPOSE: Endometrial carcinoma (EC) is a clinically heterogeneous disease characterized by a number of different histological subtypes, and its heterogeneity may be involved in the accumulation of multiple genetic alterations. The aim of this work was to investigate the comprehensive mutational profile of EC tumors, and examine the associations between somatic mutations and clinicopathological features or survival in EC patients. METHODS: A total of 100 surgical tumors were obtained from EC patients who had previously undergone surgery. Genomic DNA samples extracted from fresh-frozen tissues were analyzed using the Ion AmpliSeq Cancer Hotspot Panel v2 Kit, covering 50 tumor-related genes. RESULTS: Validated mutations were detected in 91 of the 100 tumors (91%) and identified in eight of the most frequently mutated genes, namely PTEN (57%), PIK3CA (51%), TP53 (30%), KRAS (23%), CTNNB1 (21%), FBFR2 (13%), FBXW7(10%) and RB1 (9%). PTEN mutations were found to associated with young age (< 60), early-stage, endometrioid histology, non-recurrence and better overall survival (OS). CTNNB1 mutations were associated with young age, endometrioid histology and better OS. On the other hands, TP53 mutations were associated with late-stage, non-endometrioid histology, high-grade, recurrence and worse OS. FBWX7 mutations were associated with late-stage, vascular invasion and lymph node metastasis. FGFR2 mutations correlated with deep (≥ 1/2) myometrial invasion. CONCLUSION: Our comprehensive mutational profile will be useful for understanding and evaluating the molecular characteristics of EC tumors, and may lead to the establishment of novel treatment strategies that improve the survival of patients with EC in the future.

Clinical implication of oncogenic somatic mutations in early-stage cervical cancer with radical hysterectomy.

It is well known that tumour initiation and progression are primarily an accumulation of genetic mutations. The mutation status of a tumour may predict prognosis and enable better selection of targeted therapies. In the current study, we analysed a total of 55 surgical tumours from stage IB-IIB cervical cancer (CC) patients who had undergone radical hysterectomy including pelvic lymphadenectomy, using a cancer panel covering 50 highly mutated tumorigenesis-related genes. In 35 patients (63.6%), a total 52 mutations were detected (58.3% in squamous cell carcinoma, 73.7% in adenocarcinoma), mostly in PIK3CA (34.5%) and KRAS and TP53 (9.1%). Being mutation-positive was significantly correlated with pelvic lymph node (PLN) metastasis (P = 0.035) and tended to have a worse overall survival (P = 0.076). In particular, in the patients with squamous cell carcinoma, there was a significant association between being mutation-positive and relapse-free survival (P = 0.041). The patients with PLN metastasis had a significantly worse overall survival than those without (P = 0.006). These results indicate that somatic mutation status is a predictive biomarker for PLN metastasis in early-stage CC, and is consequently related to poor prognosis. Therefore, comprehensive genetic mutations, rather than a single genetic mutation, should be examined widely in order to identify novel genetic indicators with clinical usefulness.

Tumor mutation burden and immunological, genomic, and clinicopathological factors as biomarkers for checkpoint inhibitor treatment of patients with non-small-cell lung cancer.

Cancer treatment using immune checkpoint inhibitors is widely used, although biomarkers predictive of response are not well established. However, both the expressions of programmed cell death ligand 1 (PD-L1) and the tumor mutation burden (TMB) hold promise as such biomarkers for immune checkpoint inhibitors; however, its characteristics and clinical and immunological impacts have not been fully analyzed. We, therefore, evaluated the clinical and immunological parameters related to TMB to identify potential new biomarkers. We enrolled 92 patients with non-small-cell lung cancer who underwent surgery at Fukushima Medical University Hospital from 2013 to 2016. TMB of individual tumors was calculated by whole-exome sequencing analysis. Major cancer-related gene mutations were evaluated using panel sequencing. Expression of PD-L1 and abundance of tumor-infiltrating lymphocytes were evaluated by immunohistochemistry using surgical samples. The median TMB value was 60. TMB was significantly higher in men, current or former smokers, and in patients with squamous cell carcinoma, tumor size ≥ 2.8 cm, wild-type EGFR, TP53 gene mutation-positive status, and cyclin-dependent kinase-inhibitor gene 2A mutation-positive status. According to multivariate analysis, TMB was significantly associated with EGFR gene mutation-negative status (p = 0.0111) and TP53 gene mutation-positive status (p = 0.0425). If TMB is identified as a robust biomarker for immune checkpoint inhibitor administration, analysis of TP53 and EGFR mutations may provide a relatively rapid and easy proxy for predicting TMB.

Prognostic Impact of Tumor Mutation Burden in Patients With Completely Resected Non-Small Cell Lung Cancer: Brief Report.

INTRODUCTION: Tumor mutation burden (TMB) is thought to be associated with the amount of neoantigen in the tumor and to have an important role in predicting the effect of immune checkpoint inhibitors. However, the relevance of TMB to prognosis is not yet fully understood. In this study, we investigated the clinical significance of TMB in patients with NSCLC and examined the relationship between TMB and prognosis. METHODS: We calculated TMB within individual tumors by whole-exome sequencing analysis using next-generation sequencing. We included that there were 90 patients with NSCLC who underwent surgery in the Hospital of Fukushima Medical University from 2013 to 2016. No patients received chemotherapy or immunotherapy before surgery. We assessed the correlation between TMB and prognosis. RESULTS: TMB greater than 62 was associated with worse overall survival (OS) of patients with NSCLC (hazard ratio [HR] = 6.633, p = 0.0003). Multivariate analysis showed poor prognosis with high TMB (HR = 12.31, p = 0.019). In patients with stage I NSCLC, higher TMB was associated with worse prognosis for both OS (HR = 7.582, p = 0.0018) and disease-free survival (HR = 6.07, p = 0.0072). CONCLUSIONS: High TMB in NSCLC is a poor prognostic factor. If high TMB is a predictor of the efficacy of immune checkpoint inhibitors, postoperative adjuvant therapy with immune checkpoint inhibitors may contribute to improvement of recurrence and OS.

[Exploring the Possibility of New Biomarkers of Immune Checkpoint Inhibitors for Non-Small Cell Lung Cancer (NSCLC)].

Mutation burden in a tumor, presumably involving neo-antigens in the tumor tissue, is also thought to be one of the better predictors for the efficacy of immune checkpoint inhibitors. However, it is difficult to analyze the mutation burden routinely in the clinic. Here, we describe more convenient factors that can be used as surrogate markers of mutation burden. Ninety-four patients with NSCLC who underwent resection in our institution were recruited for this study. Mutation burden and major gene alterations were analyzed by using next generation sequencing. Several immunological parameters were also assessed using immunohistochemistry. Statistical analysis was performed on mutation burden, major gene alternations, immunohistochemistry, and clinical parameters. The median mutation load was 54 mutations(range, 10-363 mutations). Squamous cell carcinoma, EGFRmutation -negativity, and TP53 alteration-positivity were closely connected with higher mutation burden. Multiple regression analysis showed that mutation burden in the tumor could be associated with EGFRmutation and TP53 alteration status.

Ribosome dimerization is essential for the efficient regrowth of Bacillus subtilis.

Ribosome dimers are a translationally inactive form of ribosomes found in Escherichia coli and many other bacterial cells. In this study, we found that the 70S ribosomes of Bacillus subtilis dimerized during the early stationary phase and these dimers remained in the cytoplasm until regrowth was initiated. Ribosome dimerization during the stationary phase required the hpf gene, which encodes a homologue of the E. coli hibernation-promoting factor (Hpf). The expression of hpf was induced at an early stationary phase and its expression was observed throughout the rest of the experimental period, including the entire 6 h of the stationary phase. Ribosome dimerization followed the induction of hpf in WT cells, but the dimerization was impaired in cells harbouring a deletion in the hpf gene. Although the absence of ribosome dimerization in these Hpf-deficient cells did not affect their viability in the stationary phase, their ability to regrow from the stationary phase decreased. Thus, following the transfer of stationary-phase cells to fresh LB medium, Δhpf mutant cells grew slower than WT cells. This observed lag in growth of Δhpf cells was probably due to a delay in restoring their translational activity. During regrowth, the abundance of ribosome dimers in WT cells decreased with a concomitant increase in the abundance of 70S ribosomes and growth rate. These results suggest that the ribosome dimers, by providing 70S ribosomes to the cells, play an important role in facilitating rapid and efficient regrowth of cells under nutrient-rich conditions.

Enhanced expression of Bacillus subtilis yaaA can restore both the growth and the sporulation defects caused by mutation of rplB, encoding ribosomal protein L2.

A temperature-sensitive mutation in rplB, designated rplB142, encodes a missense mutation at position 142 [His (CAT) to Leu (CTT)] of Bacillus subtilis ribosomal protein L2. The strain carrying the mutation grew more slowly than the wild-type, even at low temperatures, probably due to the formation of defective 70S ribosomes and the accumulation of incomplete 50S subunits (50S* subunits). Gel analysis indicated that amounts of L2 protein and also of L16 protein were reduced in ribosomes prepared from the rplB142 mutant 90 min after increasing the growth temperature to 45 °C. These results suggest that the assembly of the L16 protein into the 50S subunit requires the native L2 protein. The H142L mutation in the defective L2 protein affected sporulation as well as growth, even at the permissive temperature. A suppressor mutation that restored both growth and sporulation of the rplB142 mutant at low temperature was identified as a single base deletion located immediately upstream of the yaaA gene that resulted in an increase in its transcription. Furthermore, genetic analysis showed that enhanced synthesis of YaaA restores the functionality of L2 (H142L) by facilitating its assembly into 50S subunits.

Multiple rRNA operons are essential for efficient cell growth and sporulation as well as outgrowth in Bacillus subtilis.

The number of copies of rRNA (rrn) operons in a bacterial genome differs greatly among bacterial species. Here we examined the phenotypic effects of variations in the number of copies of rRNA genes in the genome of Bacillus subtilis by analysis of eight mutant strains constructed to carry from two to nine copies of the rrn operon. We found that a decrease in the number of copies from ten to one increased the doubling time, and decreased the sporulation frequency and motility. The maximum levels for transformation activity were similar among the strains, although the competence development was significantly delayed in the strain with a single rrn operon. Normal sporulation only occurred if more than four copies of the rrn operon were present, although ten copies were needed for vegetative growth after germination of the spores. This behaviour was seen even though the intracellular level of ribosomes was similar among strains with four to ten copies of the rrn operon. Furthermore, ten copies of the rrn operon were needed for the highest swarming activity. We also constructed 21 strains that carried all possible combinations of two copies of the rrn operons, and found that these showed a range of growth rates and sporulation frequencies that all fell between those recorded for strains with one or three copies of the rrn operon. The results suggested that the copy number of the rrn operon has a major influence on cellular processes such as growth rate and sporulation frequency.

Single mutations introduced in the essential ribosomal proteins L3 and S10 cause a sporulation defect in Bacillus subtilis.

We introduced single mutations into the rplC and rpsJ genes, which encode the essential ribosomal proteins L3 (RplC) and S10 (RpsJ), respectively, and are located in the S10 gene cluster of the gram-positive, endospore-forming bacterium Bacillus subtilis, and examined whether these mutations affected their growth rate, sporulation, competence development and 70S ribosome formation. Mutant cells harboring the G52D mutation in the L3 ribosomal protein, which is located at the peptidyl transferase center of 50S, accumulated 30S subunit at 45°C, probably due to a defect in 50S formation, and exhibited a reduction in the sporulation frequency at high temperature. On the other hand, mutant cells harboring the H56R mutation in the S10 protein, which is located near the aminoacyl-tRNA site of 30S, showed severe growth defect and deficiency in spore formation, and also exhibited significant delay in competence development.

rRNA (rrn) operon-engineered Bacillus subtilis as a feasible test organism for antibiotic discovery.

Bacillus subtilis contains 10 rRNA (rrn) operons. We found that rRNA operon-engineered B. subtilis strain RIK543, with only the rrnO operon, is specifically hypersensitive to RNA polymerase inhibitors such as rifamycin SV and rifampin (80-fold and 20-fold, respectively). In pilot screening experiments, we found actinomycete isolates successfully at an incidence of 1.9% (18/945) that produced antibacterials that were detectable only with RIK543 as the test organism. Strain RIK543 may be a feasible test organism for the discovery of novel RNA polymerase inhibitors.

An alternative sigma factor governs the principal sigma factor in Streptomyces griseus.

In bacteria, the RNA polymerase holoenzyme comprises a five-subunit core enzyme and a dissociable subunit, sigma factor, which is responsible for transcriptional initiation. The filamentous bacterium Streptomyces griseus has 52 sigma factors, including one essential 'principal' sigma factor (σ(HrdB) ) that is responsible for the transcription of housekeeping genes. Here we characterized an alternative sigma factor (σ(ShbA) ), which is highly conserved within the genus Streptomyces. A σ(ShbA) -deficient mutant showed a severe growth defect and transcriptome analysis indicated that many housekeeping genes were downregulated in response to insufficient σ(ShbA) production. Biochemical and genetic analyses proved that σ(ShbA) is a major determinant of transcription of the σ(HrdB) gene. This observation of a principal sigma factor being governed by another sigma factor throughout growth is unprecedented. We found that increasing σ(ShbA) production with mycelial growth maintained a high σ(HrdB) level late in growth. Furthermore, a hrdB-autoregulatable σ(ShbA) -deficient mutant, in which the principal sigma factor gene can be transcribed by RNA polymerase containing σ(HrdB) itself, showed several defects: rapid mycelial lysis in stationary phase in liquid culture and delayed morphological development and impaired streptomycin production in solid culture. From these observations, we discuss the biological significance of control of σ(HrdB) by σ(ShbA) in S. griseus.

Proteomic analysis of the Streptomyces griseus ribosomal fraction.

The Streptomyces griseus 70S ribosome fraction was analyzed by radical-free and highly reducing two-dimensional (RFHR 2D) gel electrophoresis and mass spectrometry. Among the 60 putative ribosomal proteins that are encoded by the S. griseus genome, 48 were identified in the 70S ribosome fraction prepared from mycelia grown in liquid culture for 12, 36, and 48 h. Ribosomal protein S3 was detected at two different positions on the 2D gel, and the distribution changed completely in the course of the growth, suggesting that it was modified or processed. The SGR3624 protein was also identified in the 70S ribosome fraction, but detailed cellular fractionation analysis indicated that it localizes mainly at the membrane rather than the ribosome. An SGR3624-deleted mutant showed slow growth on solid media, indicating that SGR3624 has an important role in the growth of the substrate mycelium in solid culture.

Expression of a small (p)ppGpp synthetase, YwaC, in the (p)ppGpp(0) mutant of Bacillus subtilis triggers YvyD-dependent dimerization of ribosome.

To elucidate the biological functions of small (p)ppGpp synthetases YjbM and YwaC of Bacillus subtilis, we constructed RIK1059 and RIK1066 strains carrying isopropyl-ß-D-thiogalactopyranoside (IPTG) inducible yjbM and ywaC genes, respectively, in the ΔrelA ΔyjbM ΔywaC triple mutant background. While the uninduced and IPTG-induced RIK1059 cells grew similarly in LB medium, the growth of RIK1066 cells was arrested following the addition of IPTG during the early exponential growth phase. Induction of YwaC expression by IPTG also severely decreased the intracellular GTP level and drastically altered the transcriptional profile in RIK1066 cells. Sucrose density gradient centrifugation analysis of the ribosomal fractions prepared from the IPTG-induced RIK1066 cells revealed three peaks corresponding to 30S, 50S, and 70S ribosome particles, and also an extra peak. Electron microscope studies revealed that the extra peak fraction contained dimers of 70S ribosomes, which were similar to the Escherichia coli 100S ribosomes. Proteomic analysis revealed that the 70S dimer contained an extra protein, YvyD, in addition to those found in the 70S ribosome. Accordingly, strain resulting from the disruption of the yvyD gene in the RIK1066 cells was unable to form 70S dimers following IPTG induction, indicating that YvyD is required for the formation of these dimers in B. subtilis.

Inactivation of ribosomal protein genes in Bacillus subtilis reveals importance of each ribosomal protein for cell proliferation and cell differentiation.

Among the 57 genes that encode ribosomal proteins in the genome of Bacillus subtilis, a Gram-positive bacterium, 50 genes were targeted by systematic inactivation. Individual deletion mutants of 16 ribosomal proteins (L1, L9, L15, L22, L23, L28, L29, L32, L33.1, L33.2, L34, L35, L36, S6, S20, and S21) were obtained successfully. In conjunction with previous reports, 22 ribosomal proteins have been shown to be nonessential in B. subtilis, at least for cell proliferation. Although several mutants that harbored a deletion of a ribosomal protein gene did not show any significant differences in any of the phenotypes that were tested, various mutants showed a reduced growth rate and reduced levels of 70S ribosomes compared with the wild type. In addition, severe defects in the sporulation frequency of the ΔrplA (L1) mutant and the motility of the ΔrpsU (S21) mutant were observed. These data provide the first evidence in B. subtilis that L1 and S21 are required for the progression of cellular differentiation.

Ribosome rescue by Escherichia coli ArfA (YhdL) in the absence of trans-translation system.

Although SsrA(tmRNA)-mediated trans-translation is thought to maintain the translation capacity of bacterial cells by rescuing ribosomes stalled on messenger RNA lacking an in-frame stop codon, single disruption of ssrA does not crucially hamper growth of Escherichia coli. Here, we identified YhdL (renamed ArfA for alternative ribosome-rescue factor) as a factor essential for the viability of E. coli in the absence of SsrA. The ssrA-arfA synthetic lethality was alleviated by SsrA(DD) , an SsrA variant that adds a proteolysis-refractory tag through trans-translation, indicating that ArfA-deficient cells require continued translation, rather than subsequent proteolysis of the truncated polypeptide. In accordance with this notion, depletion of SsrA in the ΔarfA background led to reduced translation of a model protein without affecting transcription, and puromycin, a codon-independent mimic of aminoacyl-tRNA, rescued the bacterial growth under such conditions. That ArfA takes over the role of SsrA was suggested by the observation that its overexpression enabled detection of the polypeptide encoded by a model non-stop mRNA, which was otherwise SsrA-tagged and degraded. In vitro, purified ArfA acted on a ribosome-nascent chain complex to resolve the peptidyl-tRNA. These results indicate that ArfA rescues the ribosome stalled at the 3' end of a non-stop mRNA without involving trans-translation.

Bacillus subtilis mutants harbouring a single copy of the rRNA operon exhibit severe defects in growth and sporulation.

The number of copies of rRNA genes in bacterial genomes differs greatly among bacterial species. It is difficult to determine the functional significance of the heterogeneity of each rRNA operon fully due to the existence of multiple rRNA operons and because the sequence heterogeneity among the rRNA genes is extremely low. To overcome this problem, we sequentially deleted the ten rrn operons of Bacillus subtilis and constructed seven mutant strains that each harboured a single rrn operon (either rrnA, B, D, E, I, J or O) in their genome. The growth rates and sporulation frequencies of these mutants were reduced drastically compared with those of the wild-type strain, and this was probably due to decreased levels of ribosomes in the mutants. Interestingly, the ability to sporulate varied significantly among the mutant strains. These mutants have proved to be invaluable in our initial attempts to reveal the functional significance of the heterogeneity of each rRNA operon.