Sequential replication of the Bacillus subtilis chromosome. II. Isotopic transfer experiments

Fundamental principles in bacterial physiology-history, recent progress, and the future with focus on cell size control: a review

Bacterial physiology is a branch of biology that aims to understand overarching principles of cellular reproduction. Many important issues in bacterial physiology are inherently quantitative, and major contributors to the field have often brought together tools and ways of thinking from multiple disciplines. This article presents a comprehensive overview of major ideas and approaches developed since the early 20th century for anyone who is interested in the fundamental problems in bacterial physiology. This article is divided into two parts. In the first part (sections 1-3), we review the first 'golden era' of bacterial physiology from the 1940s to early 1970s and provide a complete list of major references from that period. In the second part (sections 4-7), we explain how the pioneering work from the first golden era has influenced various rediscoveries of general quantitative principles and significant further development in modern bacterial physiology. Specifically, section 4 presents the history and current progress of the 'adder' principle of cell size homeostasis. Section 5 discusses the implications of coarse-graining the cellular protein composition, and how the coarse-grained proteome 'sectors' re-balance under different growth conditions. Section 6 focuses on physiological invariants, and explains how they are the key to understanding the coordination between growth and the cell cycle underlying cell size control in steady-state growth. Section 7 overviews how the temporal organization of all the internal processes enables balanced growth. In the final section 8, we conclude by discussing the remaining challenges for the future in the field.

The origins of 168, W23, and other Bacillus subtilis legacy strains

Bacillus subtilis is both a model organism for basic research and an industrial workhorse, yet there are major gaps in our understanding of the genomic heritage and provenance of many widely used strains. We analyzed 17 legacy strains dating to the early years of B. subtilis genetics. For three--NCIB 3610T, PY79, and SMY--we performed comparative genome sequencing. For the remainder, we used conventional sequencing to sample genomic regions expected to show sequence heterogeneity. Sequence comparisons showed that 168, its siblings (122, 160, and 166), and the type strains NCIB 3610 and ATCC 6051 are highly similar and are likely descendants of the original Marburg strain, although the 168 lineage shows genetic evidence of early domestication. Strains 23, W23, and W23SR are identical in sequence to each other but only 94.6% identical to the Marburg group in the sequenced regions. Strain 23, the probable W23 parent, likely arose from a contaminant in the mutagenesis experiments that produced 168. The remaining strains are all genomic hybrids, showing one or more "W23 islands" in a 168 genomic backbone. Each traces its origin to transformations of 168 derivatives with DNA from 23 or W23. The common prototrophic lab strain PY79 possesses substantial W23 islands at its trp and sac loci, along with large deletions that have reduced its genome 4.3%. SMY, reputed to be the parent of 168, is actually a 168-W23 hybrid that likely shares a recent ancestor with PY79. These data provide greater insight into the genomic history of these B. subtilis legacy strains.

Cell cycle dependent phosphatase activity in Bacillus subtilis

In Bacillus subtilis there is evidence that the replication origin of the DNA molecule is attached to the cytoplasmic membrane, and that this attachment is essential for the initiation of replication. We have previously shown that attachment of DNA to the membrane is cyclic and that DNA becomes attached to the cytoplasmic membrane at the beginning of replication and is subsequently released. We have also shown that membrane protein phosphorylation is associated with the initiation of DNA replication in B. subtilis. We now report that cantharidin, an inhibitor of eukaryotic serine/threonine protein phosphatases type 1 and 2A, stimulated membrane binding of DNA and caused premature initiation of DNA replication in B. subtilis cells. The membrane of B. subtilis cells had cell cycle dependent phosphatase activity against a standard substrate in vitro. Cloning of the B. subtilis protein PrpE and production of antibodies allowed us to identify the protein in the cytoplasmic membrane. Its presence was cell cycle dependent, and appeared to contribute to the observed phosphatase activity.

Cell membrane and chromosome replication in Bacillus subtilis

This review covers studies of the structural and functional roles of the cell membrane on the replication of the Bacillus subtillis chromosome. A particular emphasis is placed on the essential roles of the membrane complex for the in vivo initiation and termination of the chromosome replication. A critical gene complex in B. subtillis for the role of membrane complex is the dnaB operon that most likely consists of four genes (dnaB, dnaI, ORFZ/ORF213, and ORF omega/ORF281). Detailed studies of these genes are currently available only for the dnaB and dnaI genes. The unique feature of the dnaB gene is that temperature-sensitive mutants of this gene simultaneously lose, at the nonpermissive temperature, chromosome attachment at oriC to the membrane as well as the new round of replication initiation at oriC. Further studies on the genes and their products of the dnaB operon are therefore essential for our understanding of the in vivo mechanism of the initiation of chromosome replication and its regulation. The role of the membrane on the termination and segregation of the daughter chromosomes has not been discovered, but an important clue comes from the terminus area of the B. subtillis chromosome being bound to the membrane in a high-salt resistant and DnaB-independent fashion.

Replication fork arrest and termination of chromosome replication in Bacillus subtilis

Sporulation in Bacillus subtilis provided the first evidence for the presence of sequence-specific replication fork arrest (Ter) sites in the terminus region of the bacterial chromosome. These sites, when complexed with the replication terminator protein (RTP), block movement of a replication fork in a polar manner. The Ter sites are organized into two opposed groups which force the approaching forks to meet and fuse within a restricted terminus region. While the precise advantage provided to the cell through the presence of the so-called replication fork trap is not patently obvious, the same situation appears to have evolved independently in Escherichia coli. The molecular mechanism by which the RTP-Ter complex of B. subtilis (or the analogous but apparently unrelated complex in E. coli) functions is currently unresolved and subject to intense investigation. Replication fork arrest in B. subtilis, requiring RTP, also occurs under conditions of the stringent response at so-called STer sites that lie close to and on both sides of oriC. These sites are yet to be identified and characterized. How they are induced to function under stringent conditions is of considerable interest, and could provide vital clues about the mechanism of fork arrest by RTP-terminator complexes in general.

TRANSCRIPTION OF A REPRESSED GENE: EVIDENCE THAT IT REQUIRES DNA REPLICATION

It is known that the synthesis of DNA is not essential for constitutive or induced enzyme synthesis. Our studies indicate that DNA synthesis is required for the basal level synthesis (that is, for synthesis in absence of inducer) of two inducible enzymes, a finding which supports an earlier speculation that a messenger-RNA transcription event may normally accompany DNA replication. Studies with cultures of Escherichia coli TAU-bar in which DNA replication is synchronized suggest that the beta-galactosidase gene is transcribed at a particular time in the sequential replication of the bacterial chromosome.

UNSTABLE LINKAGE BETWEEN GENETIC MARKERS IN TRANSFORMATION

Kelly, M. (Hammersmith Hospital, London, England), and R. H. Pritchard. Unstable linkage between genetic markers in transformation. J. Bacteriol. 89:1314-1321. 1965.-A new type of association of genetic markers in a transformation system has been discovered. Linkage between certain markers is detectable only under particular conditions of deoxyribonucleic acid (DNA) extraction and is sensitive to dilution of the DNA. The data suggest that this unstable association occurs between markers which are on the same molecule but separated by a molecular distance sufficiently great for linkage to be preserved only by mild DNA extraction procedures and at high concentrations of DNA. Linkage can be stabilized by dilution in the presence of a carrier DNA. By use of these unstable linkages, the genetic map of Bacillus subtilis has been extended in both directions from the aromatic gene cluster.

Helicase-contrahelicase interaction and the mechanism of termination of DNA replication

Termination of DNA replication at a sequence-specific replication terminus is potentiated by the binding of the replication terminator protein (RTP) to the terminus sequence, causing polar arrest of the replicative helicase (contrahelicase activity). Two alternative models have been proposed to explain the mechanism of replication fork arrest. In the first model, the RTP-terminus DNA interaction simply imposes a polar barrier to helicase movement without involving any specific interaction between the helicase and the terminator proteins. The second model proposes that there is a specific interaction between the two proteins, and that the DNA-protein interaction both restricts the fork arrest to the replication terminus and determines the polarity of the process. The evidence presented in this paper strongly supports the second model.

Structure of the replication terminus-terminator protein complex as probed by affinity cleavage

The replication terminator protein (RTP) of Bacillus subtilis is a homodimer that binds to each replication terminus and impedes replication fork movement in only one orientation with respect to the replication origin. The three-dimensional structure of the RTP-DNA complex needs to be determined to understand how structurally symmetrical dimers of RTP generate functional asymmetry. The functional unit of each replication terminus of Bacillus subtilis consists of four turns of DNA complexed with two interacting dimers of RTP. Although the crystal structure of the RTP apoprotein dimer has been determined at 2.6-A resolution, the functional unit of the terminus is probably too large and too flexible to lend itself to cocrystallization. We have therefore used an alternative strategy to delineate the three dimensional structure of the RTP-DNA complex by converting the protein into a site-directed chemical nuclease. From the pattern of base-specific cleavage of the terminus DNA by the chemical nuclease, we have mapped the amino acid to base contacts. Using these contacts as distance constraints, with the crystal structure of RTP, we have constructed a model of the DNA-protein complex. The biological implications of the model have been discussed.

Identification and characterization of a novel type of replication terminator with bidirectional activity on the Bacillus subtilis theta plasmid pLS20

We have sequenced and analysed a 3.1 kb fragment of the 55 kb endogenous Bacillus subtilis plasmid pLS20 containing its replication functions. Just outside the region required for autonomous replication, a segment of 18 bp was identified as being almost identical to part of the major B. subtilis chromosomal replication terminator. Here, we demonstrate that this segment is part of a functional replication terminator. This newly identified element, designated TerLS20, is the first replication terminator identified on a theta plasmid from a Gram-positive bacterium. TerLS20 is distinct from other known replication terminators in the sense that it is functional in both orientations. The region required for bipolar functionality of TerLS20 was delineated to a sequence of 29 bp, which is characterized by an imperfect dyad symmetry.

The replication terminator protein of the gram-positive bacterium Bacillus subtilis functions as a polar contrahelicase in gram-negative Escherichia coli

The replication terminator protein (RTP) of Bacillus subtilis is a dimer with a monomeric molecular mass of 14.5 kDa. The protein terminates DNA replication at a specific binding site. Although the protein has been crystallized and its crystal structure has been solved, the lack of an in vitro replication system in B. subtilis has been a serious impediment to the analysis of the mechanism of action of this protein. We have discovered that the protein is functional in the Gram-negative bacterium Escherichia coli in vivo and in vitro. RTP blocked replication forks initiated from a ColE1 replication origin at the cognate DNA-binding site (BS3) in a polar mode. The protein did not block rolling circle replication initiated from the pT181 origin in cell extracts of Staphylococcus aureus. RTP antagonized the helicase activity of DnaB but not that of helicase II of E. coli. Thus, RTP functioned as a polar contrahelicase blocking a helicase that participates in symmetric DNA replication but it did not impede rolling circle replication nor the action of a helicase involved in DNA repair.

DNA and protein sequence conservation at the replication terminus in Bacillus subtilis 168 and W23

Cloned DNA from the replication terminus region of Bacillus subtilis 168 was used to identify and construct a restriction map of the homologous region in B. subtilis W23. With this information, DNA from the terminus region of W23 was cloned and the sequence was determined for a 1,499-base-pair segment spanning the expected terC site. The position of the site was then located more precisely. Use of the cloned DNA from strain W23 as a probe for digests of DNA from exponentially growing cells of the same strain established the presence of the slowly migrating replication termination intermediate (forked DNA). The orientation and dimensions of the forked molecule were consistent with arrest of the clockwise fork at the terC site in W23, as has been shown to occur in strain 168. Thus, despite significant differences between the two strains, the same termination mechanism appears to be used. The DNA sequences spanning the terC site in strains 168 and W23 showed a high level of homology (90.2%) close to the site but very little at a distance of approximately 250 base pairs from the site in one particular direction. The overall sequence comparison emphasised the importance of the open reading frame for a 122-amino-acid protein adjacent to terC. Although there were 22 base differences in the open reading frames between the strains, the amino acid sequence of the encoded protein was completely conserved. It is suggested that the amino acid sequence conservation reflects a role for the protein in the clockwise fork arrest mechanism as proposed earlier (M.T. Smith and R.G. Wake, J. Bacteriol. 170:4083-4090, 1988).

Genetic mapping by means of protoplast fusion in Bacillus subtilis

A new mapping method involving protoplast fusion in Bacillus subtilis is described. Protoplasts from an isogenic standard marker strain containing purA and from a strain containing both purB and the marker, "x", to be mapped were fused with polyethylene glycol, and purA+ purB+ fusants were selected. After isolation of single colonies and determination of unselected markers, marker x was mapped between two standard markers. This method was fully applicable to PBS1-resistant strains (e.g., lyt strains). The results obtained by protoplast fusion, conventional transformation and/or lysed protoplast transformation indicated that a lyt strain, Ni15, contained two new autolysin-minus mutations (lyt-151 and lyt-152). The properties of lyt-15 are also discussed.

Initiation and termination mutants of Bacillus subtilis bacteriophage SPO1

Mutants affected in cistrons 21 and 32 of bacteriophage SPO1 are defective specifically in the initiation of DNA replication. Mutations in cistron 32 also specifically affect the termination of replication.

Identification of coreplicating chromosomal sectors in Bacillus subtilis by nitrosoguanidine-induced comutation

The simultaneous replication of four regions of dichotomously replicating chromosomes of Bacillus subtilis has been detected by means of nitrosoguanidine-induced comutation. The map distance between successive rounds of replication has been measured as one-half a replicative arm in cells growing exponentially in rich medium.

Evidence for the Translocation of a Chromosome Sement in Bacillus subtilis Strains Carrying the trpE26 Mutation

The replication order of markers was studied in Bacillus subtilis strains bearing the trpE26 mutation by the use of the density transfer technique. An important difference in this order was observed in comparison with that of strain 168 T-. All markers tested of a chromosome segment extending from trpD to ilvA replicated early, after purB6 and before thr-5. Two markers flanking this region, trpE8 and citK7, replicated late as usual. The results suggested that this segment was shifted in trpE26 strains to a region closer to the origin of replication. PBS-1-mediated transduction crosses corroborated this hypothesis and revealed the position of the translocated segment. (i) Linkage was demonstrated for markers in the segment (hisH2, tryA1, met B3, ilvA2) to thr-5 and ald; (ii) aroB2 and citK7 were found to be linked; and (iii) linkage of cysB3 to thr-5 was lost in trpE26 strains. These findings made it possible to account for the characteristics of the trpE26 mutation and to propose a model explaining the fact that all trpE26+ transformants or transductants are merodiploid. The model calls for fusion of two genetic elements: two independent chromosomes, or two arms of a replicating structure. The resulting chromosome bears a long tandem duplication. Most of the features of this system of merodiploid formation can be interpreted by use of this model: the segregation pattern of the diploids, the stabilization of the unstable clones, and the length of the duplicated region. A relatively stable diploid strain was also studied by the density transfer technique. The data show that it remained diploid for the region corresponding to the translocated segment and are in agreement with the structure predicted by the model.

Revision of the linkage map of Bacillus subtilis 168: indications for circularity of the chromosome

A revision of the linkage map of the Bacillus subtilis 168 chromosome has been undertaken with the use of the generalized transducing phage PBS1. The mapping of four new markers (narB1, mtlB1, aroI906, and tre-12) has allowed a determination of the relative orientation of the purB-dal segment and its linkage with the lin markers. The chromosomal segment comprised between the sacQ36 and gtaA12 markers has been linked with the narA1, ctrA1, and sacA321 markers. The recA1 marker has been mapped relative to the thyA and citB17 markers. Indications of linkage have been found between the tre-12 and catA markers and the aroG932 and sacQ36 markers. According to these results, a circular genetic map of the chromosome of B. subtilis 168 is presented. Taken together, the transduction data and the order of marker replication determined by Harford in the accompanying paper support strongly the hypothesis of a symmetrical and fully bidirectional mode of replication for the B. subtilis 168 chromosome.

Bidirectional chromosome replication in Bacillus subtilis 168

Density transfer analysis of deoxyribonucleic acid from Bacillus subtilis 168 thy spores germinating in 5-bromouracil medium shows the order of replication of genetic markers to be: purA16, cysA14, sacA, ctrA, (narB, arol), dal, (hisA1, purB6), (tre-12, thr-5), (argA, aroG, argC4), (metC, leu-8, pheA), (ura-1, aroD), lys-1, (trpC, metB, ilvA, citB, citK, gltA). The precise order of transfer of markers within parentheses could not be determined in these experiments. Taken together with new PBS1 transduction data presented here and in the accompanying paper of J. Lepesant-Kejzlarová, J.-A. Lepesant, J. Walle, A. Billaut, and R. Dedonder (1975), the results can be resolved in terms of a symmetric, fully bidirectional mode of chromosome replication with a replication origin close to the purA16 marker and a terminus in the region of the gltA, citK loci, diametrically opposed to the origin. A new genetic map of the B. subtilis 168 chromosome is presented.

Genetic mapping in Bacillus subtilis by 5-bromouracil sensitization to ultraviolet inactivation of transforming activities

A new method for chromosome mapping of Bacillus subtilis Marburg is presented which is based on the sensitization to ultraviolet irradiation of transforming deoxyribonucleic acid that has incorporated 5-bromouracil instead of thymine. Deoxyribonucleic acid was extracted at intervals from the outgrowing spores of a thymine-requiring mutant incubated with 5-bromodeoxyuridine and subjected to a definite dose of ultraviolet irradiation. The residual activities of various genetic markers were assayed by transformation. The marker activity of deoxyribonucleic acid that had incorporated 5-bromodeoxyuridine was approximately 10 times as sensitive to ultraviolet irradiation as that of normal deoxyribonucleic acid. The markers proximal to the replication origin were sensitized at earlier times of outgrowth than distal markers. The chromosome replication in outgrowing spores was sufficiently synchronous and allowed the definite determination of when a marker became sensitized by incorporation of 5-bromodeoxyuridine. The time, designated "sensitization time," was estimated by plotting the logarithmic values of relative residual activities versus incubation times. The map constructed with sensitization times as a measurement showed good agreement with those constructed by other methods. The replication of the chromosome under the described conditions appeared to occur in the following marker order: (purA, hisA)-(purB)-(thr, pyrA)-(metC)-(leuA)-(lys, trpC, metB).

Multiple and specific initiation of T4 DNA replication

Partially replicated T4 DNA molecules (PRM) whose parental or progeny DNA was labeled with bromodeoxyuridine BUdR was analyzed by gradual shearing followed by CsCl banding of the sheared product. Analysis of PRM containing 18-mum replicated DNA showed that each replicated region was 3- to 6-mum long, indicating three to 6 replicative sites per molecule. Analysis of PRM containing 9-mum replicated DNA similarly indicated two to three replicated regions per molecule. DNA from the replicated regions of PRM containing 10-mum replicated DNA ("donor") was hybridized to DNA from mature phage ("recipient"), and the resulting hybrid was subjected to digestion with exonuclease I. The extent of protection of the recipient and more efficient self-annealing of progeny fragments from PRM indicated that the replicated regions represented 8 to 10 nonrandom locations of the genome. Possible significance of multiple sites for initiation of DNA replication is discussed.

Procedure for the isolation of mutants of Bacillus subtilis with defective cytoplasmic membranes

A procedure has been devised to isolate mutants of Bacillus subtilis with structurally defective membranes. The procedure used to screen for the mutants involved comparison of the stability of protoplasts of the mutant with those of the wild type in a medium of sufficient osmotic strength to stabilize wild-type protoplasts. Mutagenized cells were grown as clones on agar plates, and then replicated onto plates containing 0.5 m lactose, which is sufficient to stabilize wild-type protoplasts. The colonies on the lactose-containing plates were then treated with lysozyme to convert the cells to protoplasts. Colonies of wild-type protoplasts remained opaque; however, colonies of mutant protoplasts lysed and became clear. Twenty-nine osmotically fragile mutants were isolated in this manner; the membranes of several mutants were found to contain alterations in the composition of their proteins or lipids.

An osmotically fragile mutant of Bacillus subtilis with an active membrane-associated phospholipase A 1

By use of a newly developed procedure for the isolation of mutants with osmotically fragile protoplasts, a mutant of Bacillus subtilis was isolated that has a very active enzyme system for the catabolism of phospholipids via the sequential action of a phospholipase A(1) (EC 3.1.1.4) and a lysophospholipase (EC 3.1.1.5). The wild-type bacteria contain no detectable phospholipase A(1) activity, but do contain a protein that specifically inhibits the phospholipase A(1) in the mutant. This protein may play an important role in the control of phospholipid catabolism.

Structure and replication of chromosomes in competent cells of Bacillus subtilis

Competent cultures of Bacillus subtilis 168 were fractionated on gradients of Renografin-76 to obtain a population enriched for competent cells. The cells in this fraction contained two nuclear bodies. The competent cell fraction synthesized deoxyribonucleic acid and ribonucleic acid at reduced rates compared to the noncompetent cell fraction and appeared to divide synchronously upon incubation. The state of the chromosome in competent cells was determined by density transfer experiments and marker frequency analyses. The results are consistent with a competent cell possessing two, or a multiple of two, chromosomes, one complete and the other partially duplicated. During subsequent growth the partially completed chromosome replicates preferentially.

Regulation of chromosome replication in Bacillus subtilis: marker frequency analysis after amino acid starvation

Marker frequency analysis of DNA isolated from amino acid-starved Bacillus subtilis cells shows that most chromosomes have not completed replication to the terminus. This finding agrees with earlier results concerning replication after amino acid starvation in this organism. The results are not compatible with regulation of chromosome replication at the initiation step only, and they suggest that a second regulatory circuit controls replication under conditions of amino acid starvation.

Regulation of chromosome replication in Bacillus subtilis: effects of amino acid starvation in strain W23

Amino acid starvation allows limited synthesis of deoxyribonucleic acid (DNA) in Bacillus subtilis strain W23. DNA synthesis increased by about 30% after leucine starvation and by about 60% after histidine starvation. Genetic analysis on the DNA synthesized after amino acid starvation showed that all genetic markers examined have replicated, regardless of which amino acid was starved for. Initially, all markers replicated equally, but upon further replication, the thr cysB and the argA to lys regions replicated ahead of their neighboring, proximal regions. This could indicate that preferred stopping sites exist in these regions or additional sites from which replication can originate reside there. The results suggest that chromosome replication continues from those sites where it had stopped during amino acid starvation.

Symmetric replication of the Bacillus subtilis chromosome

Synchronously replicating chromosomes in germinating spores of Bacillus subtilis were labeled near the origin with [(3)H]bromouracil. The label appeared in heavy-light DNA. When cell growth and chromosome replication were continued in unlabeled bromouracil, nearly all the tritium label was transferred to heavy-heavy DNA while the terminus remained unreplicated. This implies that both origins of the replicating chromosome can undergo reinitiation. Therefore, during multifork replication the chromosome takes on the symmetric "dichotomous" form rather than the asymmetric configuration predicted by the rolling circle model.

Pre-fork synthesis: a model for DNA replication

A model of DNA replication is presented in which DNA synthesis is continuously initiated from parental strand nicks and occurs, with conservation of helix winding number, ahead of the so-called replicating fork. The fork in this model is the locus of unwinding of already replicated, but presumably unstable, DNA. The model, involving Okazaki's notion of multiple initiation, is based upon the properties of Kornberg's DNA polymerase and accounts for the presence of single-stranded nascent DNA fragments in cell lysates. In addition to acting as sites of initiation, the parental strand nicks are implicated as sites of free rotation allowing unwinding of the replicated DNA.

Synthesis of bacterial flagella: chromosomal synchrony and flagella synthesis

Synchronous cultures of Bacillus subtilis 168 M were obtained from light-density spores germinated at 46 C and grown at 37 C. This procedure synchronizes both cell division and chromosome replication. The chromosome synchrony was demonstrated by using transformation to measure changes in marker frequency during the cell cycle. The synthesis of two enzymes and of bacterial flagellar protein was also followed. All of the proteins were found to be synthesized continuously with an abrupt doubling in the rate of synthesis at a specific time in the cell cycle. The time at which the doubling occurred for each enzyme corresponded to the time at which the structural gene for the enzyme was replicated. The doubling of the rate of flagella synthesis corresponded to the time of replication of the hisA1 gene. We conclude that the genetic locus for the factors involved in the rate-limiting steps in flagella synthesis are located on the genetic map near the hisA1 locus.

Regulation of chromosome replication in Bacillus subtilis: effects of amino acid starvation in strain 168

Regulation of chromosome replication in Bacillus subtilis strain 168, in response to starvation for an essential amino acid, was found to differ from that reported for Escherichia coli. Not all replication points stop at the terminus during amino acid starvation. There is some evidence, however, to indicate that preferred stopping sites might exist. Initiation at the origin can occur in the absence of total protein synthesis as well as when the deoxyribonucleic acid (DNA)- mass ratio is unbalanced. DNA synthesis appears to be controlled independently of the initiation event by a second regulatory circuit, that may utilize the DNA-mass ratio. Once initiated, chromosome replication does not always go to completion in an uninterrupted sequence.

Chromosomal location of genes regulating resistance to bacteriophage in Bacillus subtilis

Many of the viruses which infect Bacillus subtilis require glucosylated polyglycerol teichoic acid for adsorption. These mutants can be divided into three classes on the basis of enzymatic defects and growth on galactose-minimal medium. Transduction with phage PBS1 reveals that two of these, gtaA and gtaB, are linked to hisA1, whereas the gtaC locus is linked to argC. Analysis by deoxyribonucleic acid-mediated transformation indicates that these loci exist in a cluster between the hisA1 and argC4 loci. Anomalies in mapping in the group II region of the chromosome exist. The basis of these anomalies is discussed.

Defective bacteriophage PBSH in Bacillus subtilis. II. Intracellular development of the induced prophage

Treatment of Bacillus subtilis strain 168 with mitomycin C caused induction of a defective prophage, PBSH. During induction, extensive deoxyribonucleic acid (DNA) synthesis took place. Concurrently, a change in marker frequency of the bacterial DNA was noticed. The frequency of only one marker, ade-16, the marker closest to the origin of the bacterial chromosome, was enhanced manyfold. DNA from whole phage particles transformed all bacterial markers at a frequency equal to that of DNA in the noninduced culture, except ade-16, the frequency of which was enhanced 30 to 100 times. Analysis of a double isotope experiment demonstrated that 14% of the phage DNA was derived from preinduction bacterial DNA. The other 86% of DNA in phage particles was DNA replicated after induction. Density label experiments with 5-bromodeoxyuridine showed that postinduction DNA synthesis took place preferentially at the origin region of the bacterial chromosome. Measurement of the molecular weight of DNA replicated after induction clearly showed that postinduction DNA replication is chromosomal. No evidence for prophage detachment and autonomous phage DNA replication was found. The data indicated that, after mitomycin C action, the bacterial chromosome under-went multiple reinitiation at the origin, while normal sequential DNA replication was stopped. The pool of replicated bacterial DNA was fragmented randomly. This DNA was packaged into PBSH particles which were released after cell lysis.

Development of competence in thymine-starved Bacillus subtilis with chromosomes arrested at the terminus

Tryptophan- and thymine-requiring cells of Bacillus subtilis, emerging from an amino acid starvation treatment which causes arrest of the chromosomes at the terminus, were not transformable. During subsequent incubation in a thymineless medium supplemented with amino acids, the cultures developed competence while retaining chromosome arrest. The competent subpopulation apparently shares the synchronous chromosome arrest of the bulk population. This was shown by different methods. The principal method was marker frequency analysis of the deoxyribonucleic acid extracted from a population enriched for competent cells by a column-chromatographic method. It is concluded that development of the competent state can occur in nondividing cells, and that the presence of a replication fork actively engaged in synthesis of deoxyribonucleic acid is not required for the development of this state.

Identification of conserved genetic functions in Bacillus by use of temperature-sensitive mutants

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