Stability and asymmetric replication of the Bacillus subtilis 168 chromosome structure

Lethal and mutagenic effects of different LET radiations on Bacillus subtilis spores

New, useful microorganism resources have been generated by ionizing radiation breeding technology. However, the mutagenic effects of ionizing radiation on microorganisms have not been systematically clarified. For a deeper understanding and characterization of ionizing radiation-induced mutations in microorganisms, we investigated the lethal effects of seven different linear energy transfer (LET) radiations based on the survival fraction (SF) and whole-genome sequencing analysis of the mutagenic effects of a dose resulting in an SF of around 1% in Bacillus subtilis spores. Consequently, the lower LET radiations (gamma [surface LET: 0.2 keV/µm] and 4He2+ [24 keV/µm]) showed low lethality and high mutation frequency (MF), resulting in the major induction of single-base substitutions. Whereas higher LET radiations (12C5+ [156 keV/µm] and 12C6+ [179 keV/µm]) showed high lethality and low MF, resulting in the preferential induction of deletion mutations. In addition, 12C6+ (111) ion beams likely possess characteristics of both low- and high-LET radiations simultaneously. A decrease in the relative biological effectiveness and an evaluation of the inactivation cross section indicated that 20Ne8+ (468 keV/µm) and 40Ar13+ (2214 keV/µm) ion beams had overkill effects. In conclusion, in the mutation breeding of microorganisms, it should be possible to regulate the proportions, types, and frequencies of induced mutations by selecting an ionizing radiation of an appropriate LET in accordance with the intended purpose.

An inducible recA expression Bacillus subtilis genome vector for stable manipulation of large DNA fragments

BACKGROUND

The Bacillus subtilis genome (BGM) vector is a novel cloning system based on the natural competence that enables B. subtilis to import extracellular DNA fragments into the cell and incorporate the recombinogenic DNA into the genome vector by homologous recombination. The BGM vector system has several attractive properties, such as a megabase cloning capacity, stable propagation of cloned DNA inserts, and various modification strategies using RecA-mediated homologous recombination. However, the endogenous RecA activity may cause undesirable recombination, as has been observed in yeast artificial chromosome systems. In this study, we developed a novel BGM vector system of an inducible recA expression BGM vector (iREX), in which the expression of recA can be controlled by xylose in the medium.

RESULTS

We constructed the iREX system by introducing the xylose-inducible recA expression cassette followed by the targeted deletion of the endogenous recA. Western blot analysis showed that the expression of recA was strictly controlled by xylose in the medium. In the absence of xylose, recA was not expressed in the iREX, and the RecA-mediated recombination reactions were greatly suppressed. By contrast, the addition of xylose successfully induced RecA expression, which enabled the iREX to exploit the same capacities of transformation and gene modifications observed with the conventional BGM vector. In addition, an evaluation of the stability of the cloned DNA insert demonstrated that the DNA fragments containing homologous sequences were more stably maintained in the iREX by suppressing undesirable homologous recombination.

CONCLUSIONS

We developed a novel BGM vector with inducible recA expression system, iREX, which enables us to manipulate large DNA fragments more stably than the conventional BGM vector by suppressing undesirable recombination. In addition, we demonstrate that the iREX can be applied to handling the DNA, which has several homologous sequences, such as multiple-reporter expression cassettes. Thus, the iREX expands the utility of the BGM vector as a platform for engineering large DNA fragments.

High-precision, whole-genome sequencing of laboratory strains facilitates genetic studies

Whole-genome sequencing is a powerful technique for obtaining the reference sequence information of multiple organisms. Its use can be dramatically expanded to rapidly identify genomic variations, which can be linked with phenotypes to obtain biological insights. We explored these potential applications using the emerging next-generation sequencing platform Solexa Genome Analyzer, and the well-characterized model bacterium Bacillus subtilis. Combining sequencing with experimental verification, we first improved the accuracy of the published sequence of the B. subtilis reference strain 168, then obtained sequences of multiple related laboratory strains and different isolates of each strain. This provides a framework for comparing the divergence between different laboratory strains and between their individual isolates. We also demonstrated the power of Solexa sequencing by using its results to predict a defect in the citrate signal transduction pathway of a common laboratory strain, which we verified experimentally. Finally, we examined the molecular nature of spontaneously generated mutations that suppress the growth defect caused by deletion of the stringent response mediator relA. Using whole-genome sequencing, we rapidly mapped these suppressor mutations to two small homologs of relA. Interestingly, stable suppressor strains had mutations in both genes, with each mutation alone partially relieving the relA growth defect. This supports an intriguing three-locus interaction module that is not easily identifiable through traditional suppressor mapping. We conclude that whole-genome sequencing can drastically accelerate the identification of suppressor mutations and complex genetic interactions, and it can be applied as a standard tool to investigate the genetic traits of model organisms.

In this manuscript, we describe novel applications of the newly developed Solexa sequencing technology. We aim to provide insights into the following questions: (1) Can whole-genome sequencing, while rapidly surveying mega-bases of genome information, also reliably identify variations at the base-pair resolution? (2) Can it be used to identify the differences between isolates of the same laboratory strain and between different laboratory strains? (3) Can it be used as a genetic tool to predict phenotypes and identify suppressors? To this end, we performed whole-genome shotgun sequencing of several related strains of the widely studied model bacterium Bacillus subtilis, we identified genomic variations that potentially underlie strain-specific phenotypes, which occur frequently in biological studies, and we found multiple suppressor mutations within a single strain that are difficult to discern through traditional methods. We conclude that whole-genome sequencing can be directly used to guide genetic studies.

Combining two genomes in one cell: stable cloning of the Synechocystis PCC6803 genome in the Bacillus subtilis 168 genome

Cloning the whole 3.5-megabase (Mb) genome of the photosynthetic bacterium Synechocystis PCC6803 into the 4.2-Mb genome of the mesophilic bacterium Bacillus subtilis 168 resulted in a 7.7-Mb composite genome. We succeeded in such unprecedented large-size cloning by progressively assembling and editing contiguous DNA regions that cover the entire Synechocystis genome. The strain containing the two sets of genome grew only in the B. subtilis culture medium where all of the cloning procedures were carried out. The high structural stability of the cloned Synechocystis genome was closely associated with the symmetry of the bacterial genome structure of the DNA replication origin (oriC) and its termination (terC) and the exclusivity of Synechocystis ribosomal RNA operon genes (rrnA and rrnB). Given the significant diversity in genome structure observed upon horizontal DNA transfer in nature, our stable laboratory-generated composite genome raised fundamental questions concerning two complete genomes in one cell. Our megasize DNA cloning method, designated megacloning, may be generally applicable to other genomes or genome loci of free-living organisms.

Diversity of genome structure in Salmonella enterica serovar Typhi populations

The genomes of most strains of Salmonella and Escherichia coli are highly conserved. In contrast, all 136 wild-type strains of Salmonella enterica serovar Typhi analyzed by partial digestion with I-CeuI (an endonuclease which cuts within the rrn operons) and pulsed-field gel electrophoresis and by PCR have rearrangements due to homologous recombination between the rrn operons leading to inversions and translocations. Recombination between rrn operons in culture is known to be equally frequent in S. enterica serovar Typhi and S. enterica serovar Typhimurium; thus, the recombinants in S. enterica serovar Typhi, but not those in S. enterica serovar Typhimurium, are able to survive in nature. However, even in S. enterica serovar Typhi the need for genome balance and the need for gene dosage impose limits on rearrangements. Of 100 strains of genome types 1 to 6, 72 were only 25.5 kb off genome balance (the relative lengths of the replichores during bidirectional replication from oriC to the termination of replication [Ter]), while 28 strains were less balanced (41 kb off balance), indicating that the survival of the best-balanced strains was greater. In addition, the need for appropriate gene dosage apparently selected against rearrangements which moved genes from their accustomed distance from oriC. Although rearrangements involving the seven rrn operons are very common in S. enterica serovar Typhi, other duplicated regions, such as the 25 IS200 elements, are very rarely involved in rearrangements. Large deletions and insertions in the genome are uncommon, except for deletions of Salmonella pathogenicity island 7 (usually 134 kb) from fragment I-CeuI-G and 40-kb insertions, possibly a prophage, in fragment I-CeuI-E. The phage types were determined, and the origins of the phage types appeared to be independent of the origins of the genome types.

Study of chromosome rearrangements associated with the trpE26 mutation of Bacillus subtilis

Chromosome rearrangements involved in the formation of merodiploid strains in the Bacillus subtilis 168-166 system were explained by postulating the existence of intrachromosomal homology regions. This working hypothesis was tested by analysing sequences and restriction patterns of the, as yet uncharacterized, junctions between chromosome segments undergoing rearrangements in parent, 168 trpC2 and 166 trpE26, as well as in derived merodiploid strains. Identification, at the Ia/Ib chromosome junction of both parent strains, of a 1.3 kb segment nearly identical to a segment of prophage SPbeta established the existence of one of the postulated homology sequences. Inspection of relevant junctions revealed that a set of different homology regions, derived from prophage SPbeta, plays a key role in the formation of so-called trpE30, trpE30+, as well as of new class I merodiploids. Analysis of junctions involved in the transfer of the trpE26 mutation, i.e. simultaneous translocation of chromosome segment C and rotation of the terminal relative to the origin moiety of the chromosome, did not confirm the presence of any sequence suitable for homologous recombination. We propose a model involving simultaneous introduction of four donor DNA molecules, each comprising a different relevant junction, and their pairing with the junction regions of the recipient chromosome. The resolution of this structure, resting on homologous recombination, would confer the donor chromosome structure to the recipient, achieving some kind of 'transstamping'. In addition, a rather regular pattern of inverse and direct short sequence repeats in regions flanking the breaking points could be correlated with the initial, X-ray-induced, rearrangement.

Isolation of RNase H genes that are essential for growth of Bacillus subtilis 168

Two genes encoding functional RNase H (EC 3.1.26.4) were isolated from a gram-positive bacterium, Bacillus subtilis 168. Two DNA clones exhibiting RNase H activities both in vivo and in vitro were obtained from a B. subtilis DNA library. One (28.2 kDa) revealed high similarity to Escherichia coli RNase HII, encoded by the rnhB gene. The other (33.9 kDa) was designated rnhC and encodes B. subtilis RNase HIII. The B. subtilis genome has an rnhA homologue, the product of which has not yet shown RNase H activity. Analyses of all three B. subtilis genes revealed that rnhB and rnhC cannot be simultaneously inactivated. This observation indicated that in B. subtilis both the rnhB and rnhC products are involved in certain essential cellular processes that are different from those suggested by E. coli rnh mutation studies. Sequence conservation between the rnhB and rnhC genes implies that both originated from a single ancestral RNase H gene. The roles of bacterial RNase H may be indicated by the single rnhC homologue in the small genome of Mycoplasma species.

Genetic transfer of large DNA inserts to designated loci of the Bacillus subtilis 168 genome

It was found that contiguous DNA segments of up to 50 kb can be transferred between Bacillus subtilis genomes when a sufficient length of the flanking genomic region is provided for homologous recombination, although the efficiency of transfer was reduced as the insert size increased. Inserts were translocated to different loci, where appropriate integration sites were created.

Physical map of the Bacillus subtilis 166 genome: evidence for the inversion of an approximately 1900 kb continuous DNA segment, the translocation of an approximately 100 kb segment and the duplication of a 5 kb segment

An I-CeuI-NotI-SfiI endonuclease map of the Bacillus subtilis 166 genome was constructed. It was almost identical to that of B. subtilis 168 except for the inversion of an approximately 1900 kb DNA segment, the translocation of an approximately 100 kb segment and the duplication of a 5 kb segment. Continuity of the inverted segment was investigated by direct measurement of the distances between the two genomic loci where I-SceI recognition sites were created in the 168 and the 166 genomes. Size difference of the I-SceI fragments between the two strains fully demonstrated the inversion of an approximately 1900 kb long 'continuous' DNA segment and the 'location' of the two inversion junctions in the genome. The 100 kb DNA segment including the lysogenic SP beta prophage was translocated close to one of the inversion junctions and was probably associated with the duplication of a 5 kb segment. These rearrangements are consistent with those indicated by genetic analyses.

The complete genome sequence of the gram-positive bacterium Bacillus subtilis

Bacillus subtilis is the best-characterized member of the Gram-positive bacteria. Its genome of 4,214,810 base pairs comprises 4,100 protein-coding genes. Of these protein-coding genes, 53% are represented once, while a quarter of the genome corresponds to several gene families that have been greatly expanded by gene duplication, the largest family containing 77 putative ATP-binding transport proteins. In addition, a large proportion of the genetic capacity is devoted to the utilization of a variety of carbon sources, including many plant-derived molecules. The identification of five signal peptidase genes, as well as several genes for components of the secretion apparatus, is important given the capacity of Bacillus strains to secrete large amounts of industrially important enzymes. Many of the genes are involved in the synthesis of secondary metabolites, including antibiotics, that are more typically associated with Streptomyces species. The genome contains at least ten prophages or remnants of prophages, indicating that bacteriophage infection has played an important evolutionary role in horizontal gene transfer, in particular in the propagation of bacterial pathogenesis.

Experimental surgery to create subgenomes of Bacillus subtilis 168

The 4,188-kb circular genome of Bacillus subtilis 168 was artificially dissected into two stable circular chromosomes in vivo, one being the 3,878-kb main genome and the other the 310-kb subgenome that was recovered as covalently closed circular DNA in CsCl-ethidium bromide ultracentrifugation. The minimal requirements to physically separate the 310-kb DNA segment out of the genome were two interrepeat homologous sequences and an origin of DNA replication between them. The subgenome originated from the 1,255-1, 551-kb region of the B. subtilis genome was essential for the cell to survive because the subgenome was not lost from the cell. The finding that the B. subtilis genome has a potential to be divided and the resulting two replicons stably maintained may shed light on origins and formation mechanisms of giant plasmids or second chromosomes present in many bacteria. Similar excision or its reversal process, i.e., integration of large sized covalently closed circular DNA pieces into the main genome, implies significant roles of subgenomes in the exchange of genetic information and size variation of bacterial genomes in bacterial evolution.

Diverse capacities for the adaptive response to DNA alkylation in Bacillus species and strains

Our previous studies of Bacillus subtilis showed that the genes responsible for the adaptive response to DNA alkylation were organized as a divergent regulon, in contrast to scattered operons in Escherichia coli ada regulon. To study the generality and diversity of gene organization, several species and strains of Bacillus were examined for the responsiveness to DNA alkylation. B. cereus cells exhibited the highest resistance to MNNG treatment. When the cells were grown in the presence of MNNG, 3-methyladenine DNA glycosylase and two species of DNA methyltransferase were induced as in B. subtilis 168 cells. B. licheniformis 749 and B. amyloliquefaciens H cells exhibited a partial response that manifested itself as the induction of one species of DNA methyltransferase. On the other hand, B. thuringiensis var. Tohokuensis, B. megaterium KMT, and B. subtilis W23 cells were totally deficient in this response, and were hypersensitive to alkylating agents. To determine the cause of this deficiency in strain W23, we examined the genomic structure of the corresponding region where three genes (alkA, adaA, and adaB) were located in 168. No homologues for the three genes were detected in W23 DNA by Southern hybridization. Two genes (glmS and ndhF) flanking the adaptive response regulon in 168 were also present in W23. A sequence of about 2750 bp that carried the entire regulon in 168 was replaced with a sequence of about 250 bp that was unique to W23. At the ends of the conserved segments, palindromic sequences corresponding to the transcriptional termination sites of the adaB and glmS genes were observed. The regulon in 168 could be artificially replaced by the W23 sequence, and be regained through DNA-mediated transformation.

I-CeuI recognition sites in the rrn operons of the Bacillus subtilis 168 chromosome: inherent landmarks for genome analysis

The Bacillus subtilis 168 circular chromosome yielded ten fragments on I-CeuI endonuclease digestion. I-CeuI recognizes a 26 bp sequence that is located within the gene encoding the 23S subunit of the rRNA in Chlamydomonas eugametos, Escherichia coli and Salmonella typhimurium. The precise locations of the I-CeuI sites of the B. subtilis chromosome were determined on a NotI-SfiI physical map by (i) double digestion analyses with I-CeuI and SfiI, (ii) comparison of mutant strains lacking a specific rrn operon, (iii) using an I-CeuI linking clone and (iv) analysis of nucleotide sequence data of some rrn operons. In conclusion, all the I-CeuI sites were located within the B. subtilis rrn operons and the I-CeuI sites were conserved in all the B. subtilis 168 derivatives tested. Thus, variations in size of the I-CeuI fragments must be due to genome alterations. A B. subtilis 168 strain was investigated with I-CeuI. We demonstrated that the aberrant structure was the outcome of the inversion of an approximately 1700 kb DNA segment.

Toward a bacterial genome technology: integration of the Escherichia coli prophage lambda genome into the Bacillus subtilis 168 chromosome

A novel approach to the cloning large DNAs in the Bacillus subtilis chromosome was examined. An Escherichia coli prophage lambda DNA (48.5 kb) was assembled in the chromosome of B. subtilis. The lambda DNA was first subcloned in four segments, having partially overlapping regions. Assembly of the complete prophage was achieved by successive transformation using three discrete DNA integration modes: overlap-elongation, Campbell-type integration, and gap-filling. In the B. subtilis chromosome, DNA was elongated, using contiguous DNA segments, via overlap-elongation. Jumping from one end of a contiguous DNA stretch to another segment was achieved by Campbell-type integration. The remaining gap was sealed by gap-filling. The incorporated lambda DNA thus assembled was stably replicated as part of the 4188 kb B. subtilis chromosome under non-selective conditions. The present method can be used to accommodate larger DNAs in the B. subtilis chromosome and possible applications of this technique are discussed.

A Bacillus subtilis spore coat polypeptide gene, cotS

A gene, cotS, encoding a spore coat polypeptide of Bacillus subtilis, was isolated from an EcoRI fragment (5.4 kb) of the chromosome by using synthetic oligonucleotide probes corresponding to the NH2-terminal amino acid sequence of Cot40-2 previously purified from the spore coat of B. subtilis. The nucleotide sequence (2603 bp) was determined and sequence analysis suggested the presence of two contiguous ORFs, ORF X and cotS, followed by the 5'-region of an additional ORF, ORF Y, downstream of cotS. The cotS gene is 1053 nucleotides long and encodes a polypeptide of 351 amino acids with a predicted molecular mass of 41083 Da. The predicted amino acid sequence was in complete agreement with the NH2-terminal amino acid sequence of Cot40-2. The orfX gene is 1131 nucleotides long and encodes a polypeptide of 377 amino acids with a predicted molecular mass of 42911 Da. The gene product of cotS was confirmed to be identical to Cot40-2 by SDS-PAGE and immunoblotting from Escherichia coli transformed with a plasmid containing the cotS region. Northern hybridization analysis indicated that a transcript of cotS and orfX appeared at about 5 h after the onset of sporulation. The transcriptional start point determined by primer extension analysis suggested that -10 and -35 regions are present upstream of orfX and are very similar to the consensus sequence for the sigma k-dependent promoter. Terminator-like sequences were not found in the DNA fragment (2603 bp) sequenced in this paper, which suggested that the cotS locus may be part of a multicistronic operon. The cotS gene is located between dnaB and degQ at about 270-275 degrees on the genetic map. Insertional mutagenesis of the cotS gene by introducing an integrative plasmid resulted in no alteration of growth or sporulation, and had no effect on germination or resistance to chloroform.

An estimation of minimal genome size required for life

The number of indispensable chromosomal loci for a bacterium, Bacillus subtilis was estimated. Seventy-nine randomly selected chromosomal loci were investigated by mutagenesis. Mutation at only six loci rendered B. subtilis unable to form colonies. In contrast, mutants for the rest of the 73 loci retained the ability to form colonies. Mutant B. subtilis with multiple-fold mutations of those dispensable loci (7-, 12- or 33-fold) were not impaired in their ability to form colonies on nutritionally adequate medium, indicating that up to 33 dispensable loci were simultaneously abolished. Given the statistical analyses for the frequency of indispensable loci (6 out of 79), total indispensable genetic material would be included within about 562 kbp. The hypothetical minimum genome size lies in the range of those currently determined smallest genomes for bacteria.

Integration of repeated sequences (pBR322) in the Bacillus subtilis 168 chromosome without affecting the genome structure

The Escherichia coli plasmid pBR322 sequence (4363 bp) was integrated at the met, pro, or leuB locus of the Bacillus subtilis chromosome without duplication of the flanking chromosomal regions. The integrated pBR322 was stably maintained as part of the chromosome regardless of its orientation or location. It was found that a DNA segment as large as 17 kb cloned in pBR322 can be readily transferred to the B. subtilis chromosome by transformation. It was demonstrated that a second pBR322 sequence could be effectively introduced at different regions of the chromosome by sequential transformation using chromosomal DNA isolated from a strain that had already acquired a pBR322 sequence at a different locus. Similarly, a third pBR322 sequence could be introduced. By this method, two or three pBR322 sequences can be incorporated at unlinked loci without affecting the overall structure of the B. subtilis genome.