Uptake of circular deoxyribonucleic acid and mechanism of deoxyribonucleic acid transport in genetic transformation of Streptococcus pneumoniae

Clp ATPases differentially affect natural competence development in Streptococcus mutans

In naturally competent bacteria, DNA transformation through horizontal gene transfer is an evolutionary mechanism to receive extracellular DNA. Bacteria need to maintain a state of competence to accept foreign DNA, and this is an energy-driven phenomenon that is tightly controlled. In Streptococcus, competence development is a complex process that is not fully understood. In this study, we used Streptococcus mutans, an oral bacterium, to determine how cell density affects competence development. We found that in S. mutans the transformation efficiency is maximum when the transforming DNA was added at low cell density and incubated for 2.5 h before selecting for transformants. We also found that S. mutans cells remain competent until the mid-logarithmic phase, after which the competence decreases drastically. Surprisingly, we observed that individual components of Clp proteolytic complexes differentially regulate competence. If the transformation is carried out at the early growth phase, both ClpP protease and ClpX ATPase are needed for competence. In contrast, we found that both ClpC and ClpE negatively affect competence. We also found that if the transformation is carried out at the mid-logarithmic growth phase ClpX is still required for competence, but ClpP negatively affects competence. While the exact reason for this differential effect of ClpP and ClpX on transformation is currently unknown, we found that both ClpC and ClpE have a negative effect on transformation, which was not reported before.

An unstable competence-induced protein, CoiA, promotes processing of donor DNA after uptake during genetic transformation in Streptococcus pneumoniae

Natural genetic transformation in Streptococcus pneumoniae entails transcriptional activation of at least two sets of genes. One set of genes, activated by the competence-specific response regulator ComE, is involved in initiating competence, whereas a second set is activated by the competence-specific alternative sigma factor ComX and functions in DNA uptake and recombination. Here we report an initial characterization of CoiA, a ComX-dependent gene product that is induced during competence and is required for transformation. CoiA is widely conserved among gram-positive bacteria, and in streptococci, the entire coiA locus composed of four genes is conserved. By use of immunoblot assay, we show that, similar to its message, CoiA protein is transient, appearing at 10 min and largely disappearing by 30 min post-competence induction. Using complementation analysis, we establish that coiA is the only gene of this induced locus needed for transformability. We find no indication of CoiA having a role in regulating competence. Finally, using 32P- and 3H-labeled donor DNA, we demonstrate that a coiA mutant can internalize normal amounts of donor DNA compared to the wild-type strain but is unable to process it into viable transformants, suggesting a role for CoiA after DNA uptake, either in DNA processing or recombination.

Evolutionary stability of DNA uptake signal sequences in the Pasteurellaceae

The DNA-uptake signal sequence (USS) of the bacterium Haemophilus influenzae is highly over-represented in its genome (1,471 copies of the core sequence AAGTGCGGT), and DNA fragments containing USS are preferentially taken up by competent cells. Because this bias favors uptake of conspecific DNA, USSs are often considered a kind of mate recognition system in bacteria, acting as species-specific barriers against uptake of unrelated DNA. However, the H. influenzae USS is highly over-represented in the genomes of three otherwise-divergent Pasteurellaceae species (Pasteurella multocida, Haemophilus somnus, and Actinobacillus actinomycetemcomitans, 927, 1,205, and 1,760 copies, respectively), suggesting that USSs do not always limit exchange. USSs in all these genomes are mainly in coding regions and show no orientation bias around the chromosome, weakening proposed USS functions in transcription termination and chromosome replication. Alignment of homologous genes was used to determine evolutionary relationships between individual USSs. Most H. influenzae USSs were found to have perfect or imperfect homologs (USS at the same location) in at least one other species, and most USSs in the other species had perfect or imperfect homologs in H. influenzae. These homologies suggest that the use of a common USS is due to inheritance of the USS-based uptake system from a common ancestor of the Pasteurellaceae, and it indicates that individual USSs can be evolutionarily stable elements of their genomes. The pattern is consistent with a molecular drive model of USS evolution, with new USSs arising by mutation and preferentially spread to new genomes by the biased DNA-uptake system.

Constitutive competence for genetic transformation in Streptococcus pneumoniae caused by mutation of a transmembrane histidine kinase

Competence for DNA uptake and genetic transformation in Streptococcus pneumoniae is regulated by a quorum-sensing system. A competence-stimulating polypeptide (CSP) is secreted by the bacteria and acts back on the cells via a transmembrane histidine kinase. This enzyme phosphorylates a response regulator that activates synthesis of a SigH-like protein. The new sigma factor enables expression of a set of proteins transcribed from a novel promoter. A mutation called trt had been found that circumvented this regulation. The mutant cells are constitutively competent; that is, they can be transformed at low cell densities, in the presence of proteases that attack CSP, or during growth at low pH. In this work, cells containing trt were shown to be competent even in the presence of a comAB mutation that blocks secretion of CSP. The trt mutation was localized to comD, the gene encoding the transmembrane histidine kinase. A DNA segment of the trt mutant corresponding to comCDE was cloned, and it was shown to contain the trt mutation by its ability to confer constitutive competence. A two-step assay, which was based on transfer of trt to a wild strain and screening for transformability in the presence of trypsin, served to locate the trt mutation precisely. It corresponds to a GC-->AT transition, which changes Asp299 in the histidine kinase to Asn. This alteration in the carboxyl terminal half of the protein, which is cytoplasmically located and contains the phosphorylase activity, presumably alters the enzyme conformation so that it is permanently activated, independent of signals from the transmembrane domain. These results may help illuminate the mechanism by which external signals affect kinase action in two-component regulatory systems, and they may be of practical value in facilitating genetic studies by rendering pneumococcal strains permanently competent.

Cloning and expression of pneumococcal genes in Streptococcus pneumoniae

An overview of gene cloning in Streptococcus pneumoniae is presented. The advantages of such cloning, especially for pneumococcal genes, are enumerated. The molecular fate of DNA in transformation of S. pneumoniae, in particular, the conversion of DNA to single-strand segments on entry, determines the mechanisms for plasmid establishment and interaction with the chromosome. One of these mechanisms, the chromosomal facilitation of plasmid establishment, is useful for obtaining recombinant plasmids and for introducing an allele from the chromosome into a plasmid. The difference between linear and circular synapsis of donor DNA strands with the chromosome is illustrated. Circular synapsis can give rise to circular integration, which is useful for insertional mutagenesis of chromosomal genes, for coupled cloning in Escherichia coli, and for sequential cloning of DNA along the pneumococcal chromosome. Cloning in S. pneumoniae is not notably affected by DNA mismatch repair or restriction systems in the host cell. Unusual features of gene expression in S. pneumoniae are discussed. Transcription begins most often at promoters with extended -10 sequences, and in a small but significant number of cases, translation does not require a ribosome-binding site with a Shine-Dalgarno sequence.

DNA uptake in bacteria

Natural competence is widespread among bacterial species. The mechanism of DNA uptake in both gram-positive and gram-negative bacteria is reviewed. The transformation pathways are discussed, with attention to the fate of donor DNA as it is processed by the competent cell. The proteins involved in mediating various steps in these pathways are described, and models for the transformation mechanisms are presented. Uptake of DNA across the inner membrane is probably similar in gram-positive and gram-negative bacteria, and at least some of the required proteins are orthologs. The initial transformation steps differ, as expected, from the presence of an outer membrane only in the gram-negative organisms. The similarity of certain essential competence proteins to those required for the assembly of type-4 pili and for type-2 protein secretion is discussed. Finally several hypotheses for the biological role of transformation are presented and evaluated.

Polarity of recombination in transformation of Streptococcus pneumoniae

In transformation of Streptococcus pneumoniae DNA enters the cell as single-strand fragments and integrates into the chromosome by homologous recombination. Deletions and insertions of a few hundred base pairs frequently stop the recombination process of a donor strand. In this work we took advantage of such interruptions of recombination to compare the transformation efficiencies of the segments 5'- and 3'-ward from a deletion. The deletion was created in the center of a fragment of the ami locus, and sites around the deletion were labeled by a frameshift generating a restriction site. Heteroduplexes were constructed containing two restriction sites on one strand and two different ones on the complementary strand. ami+ bacteria were transformed with such heteroduplexes. ami- transformants were isolated and individually underwent amplification of the transformed ami region. We have obtained two kinds of amplification products: short when the deletion was integrated, long when recombination stops at the deletion. Each long fragment was tested by the four restriction enzymes to detect which strand and which side of the deletion had recombined. We found that 80% of the cuts were located 5' to the deletion, showing that, in vivo, the 5' side is strongly favored by recombination. Further results suggest that exchanges occurring from 5' to 3' relative to the donor strand are more efficient than in the opposite direction, thus accounting for the 5' preference.

Multiple roles for DNA polymerase I in establishment and replication of the promiscuous plasmid pLS1

The polymerase activity of DNA polymerase I is important for the establishment of the pLS1 replicon by reconstitutive assembly in Streptococcus pneumoniae after uptake of exogenous pLS1 plasmid DNA. In polA mutants lacking the polymerase domain, such establishment was reduced at least 10-fold in frequency. Chromosomally facilitated establishment of pLS1-based plasmids carrying DNA homologous to the host chromosome was not so affected. However, both types of plasmid transfer gave mostly small colonies on initial selection, which was indicative of a defect in replication and filling of the plasmid pool. Once established, the pLS1-based plasmids replicated in polA mutants, but they showed segregational instability. This defect was not observed in strains with the wild-type enzyme or in an S. pneumoniae strain that encodes the polymerase and exonuclease domains of the enzyme on separate fragments. The role of DNA polymerase I in stably maintaining the plasmids depends on its polymerizing function in three separate steps of rolling-circle replication, as indicated by the accumulation of different replication intermediate forms in polA mutants. Furthermore, examination of the segregational stability of the pLS1 replicon in an Escherichia coli mutant system indicated that both the polymerase and the 5'-to-3' exonuclease activities of DNA polymerase I function in plasmid replication.

Human T-helper cell recognition of an immunodominant epitope of HIV-1 gp120 expressed on the surface of Streptococcus gordonii

Our genetic system for expression of heterologous proteins on the surface of the Gram-positive bacterium Streptococcus gordonii was used to express a human T-helper epitope of HIV-1 envelope glycoprotein gp120. In previous work on the naive repertoire of human T-helper cells, it was shown that a 15-amino acid synthetic peptide of the HIV-1 gp120 sequence contained an immunodominant T-helper epitope. Synthetic DNA coding for this peptide was cloned in frame within the gene for the streptococcal surface protein M6, and the gene fusion was integrated by transformation into the chromosome of S. gordonii. The expected M6-gp120 fusion protein was found to be expressed on the surface of the recombinant streptococci. To test whether the T epitope could be recognized by T cells when expressed on the bacterial surface within the context of M6, recombinant bacteria were used as antigen in proliferation assays to stimulate the 15-amino acid-specific human T-helper clone, in the presence of autologous antigen-presenting cells. Bacteria expressing the T epitope were efficiently recognized by the T cells in culture. In proliferation assays, 10(6)-10(7) bacteria induced responses comparable to those obtained by standard amounts of synthetic peptide (0.02-0.2 micrograms). Recombinant S. gordonii, a candidate for a live vaccine vector, appeared suitable for delivering T epitopes to the immune system.

Bioenergetic aspects of the translocation of macromolecules across bacterial membranes

Bacteria are extremely versatile in the sense that they have gained the ability to transport all three major classes of biopolymers through their cell envelope: proteins, nucleic acids, and polysaccharides. These macromolecules are translocated across membranes in a large number of cellular processes by specific translocation systems. Members of the ABC (ATP binding cassette) superfamily of transport ATPases are involved in the translocation of all three classes of macromolecules, in addition to unique transport ATPases. An intriguing aspect of these transport processes is that the barrier function of the membrane is preserved despite the fact the dimensions of the translocated molecules by far surpasses the thickness of the membrane. This raises questions like: How are these polar compounds translocated across the hydrophobic interior of the membrane, through a proteinaceous pore or through the lipid phase; what drives these macromolecules across the membrane; which energy sources are used and how is unidirectionality achieved? It is generally believed that macromolecules are translocated in a more or less extended, most likely linear form. A recurring theme in the bioenergetics of these translocation reactions in bacteria is the joint involvement of free energy input in the form of ATP hydrolysis and via proton sym- or antiport, driven by a proton gradient. Important similarities in the bioenergetic mechanisms of the translocation of these biopolymers therefore may exist.

Expression of foreign proteins on gram-positive commensal bacteria for mucosal vaccine delivery

Non-pathogenic Gram-positive oral commensal bacteria expressing recombinant fusion proteins on their cell surface have been successfully used to raise both a mucosal and a systemic immune response to foreign antigens while colonizing the oropharynx. In this system, fusion-protein vaccines are delivered and anchored to the surface of a commensal, which occupies the mucosal niche invaded by a particular pathogen. Surface expression of these foreign proteins is achieved by exploiting the common mechanism employed by Gram-positive bacteria for translocating and anchoring proteins to the cell surface. The process offers a safe alternative to the use of engineered pathogens as live vaccine delivery vehicles.

Direction of DNA entry in competent cells of Bacillus subtilis

Direction of DNA entry in Bacillus subtilis competent cells was studied using molecules in which only one of the two strands was radioactively labelled. The label was either distributed homogeneously or was localized in a small region of the strand, in the centre or at one of the ends. Regardless of the distribution and the position of the label, similar amounts of radioactivity were taken up by the cells exposed to the labelled molecules. This suggests that DNA enters B. subtilis either by two different uptake systems having opposite polarities, or by a single non-polar system.

Polarity of DNA entry in transformation of Streptococcus pneumoniae

DNA transport in Streptococcus pneumoniae was studied using donor molecules labelled either at the 3' or at the 5' end, on one strand only. In contrast to 5' end label, 3' end label was not taken up by the cells indicating that entry is a polarized process. Our results together with those of previous studies are consistent with a model for entry in which double-stranded donor DNA is nicked on binding at the cell surface. Entry of a single strand then proceeds linearly from a newly formed 3' end to the extremity of the donor fragment.

Linearization of donor DNA during plasmid transformation in Neisseria gonorrhoeae

We examined the fate of plasmid DNA after uptake during transformation in Neisseria gonorrhoeae. An 11.5-kilobase plasmid, pFA10, was processed to linear double-stranded DNA during uptake by competent cells, but cleavage of pFA10 was not site specific. A minority of pFA10 entered as open circles. A 42-kilobase plasmid, pFA14, was degraded into small fragments during uptake; no intracellular circular forms of pFA14 were evident. Since pFA10 DNA linearized by a restriction enzyme was not further cut during uptake, the endonucleolytic activity associated with entry of plasmid DNA appeared to act preferentially on circular DNA. Although linear plasmid DNA was taken up into a DNase-resistant state as efficiently as circular DNA, linear plasmid DNA transformed much less efficiently than circular plasmid DNA. These data suggest that during entry transforming plasmid DNA often is processed to double-stranded linear molecules; transformants may arise when some molecules are repaired to form circles. Occasional molecules which enter as intact circles may also lead to transformants.

Homology-facilitated plasmid transfer in Haemophilus influenzae

The 8 kbp plasmid pAT4 transformed Haemophilus influenzae Rd cells at low frequencies. Transformation was increased up to 100 times, however, when the recipient cells carried a DNA segment in either their chromosome or in a resident plasmid that was homologous to at least part of plasmid pAT4. Linearized plasmid DNA molecules did not transform cells without DNA homology; they efficiently transformed homology recipients, but only when the cuts had been made in the region of shared homology. In most cases examined the circular donor plasmid had been reconstituted from the transforming DNA; in some cases the reconstituted plasmid carried a mutation initially present in the recipient chromosome, provided the transforming plasmid had been linearized in the region of shared homology. Plasmid reconstitution was not observed in recA1 cells. We conclude that homology-facilitated plasmid transformation (transfer) is similar to that reported for Bacillus subtilis and Streptococcus pneumoniae.

Reexamination of phenotypic defects in rec-1 and rec-2 mutants of Haemophilus influenzae Rd

Radiolabeled donor DNA is efficiently taken up into competent H. influenzae Rd rec-2 mutant cells but does not undergo the rapid degradation observed in wild-type cells. Furthermore, donor label is not recovered in the chromosome even after 1 h. The donor DNA appears to remain in a protected state in a compartment that can be separated from the rest of the cell. We interpret this as a failure of the donor DNA to be translocated out of the transformasome. In contrast, rec-1 cells translocate labeled donor DNA normally. The donor label accumulates in the recipient chromosome, but, as expected for cells with a recombination defect, there is no preferential localization of the label in sites homologous to the donor DNA. In addition, we have observed two enzymatic activities that act on transformasome-associated DNA of rec-2 cells, an endonuclease which may play a role in the translocation of closed circular DNA and a phosphatase.

Molecular cloning of the lactose-metabolizing genes from Streptococcus lactis

Restriction endonucleases and agarose gel electrophoresis were used to analyze plasmid pLM2001, which is required for lactose metabolism by Streptococcus lactis LM0232. The enzymes XhoI, SstI, BamHI, and KpnI each cleaved the plasmid into two fragments, whereas EcoRI and BglII cleaved the plasmid into seven and five fragments, respectively. Sizing of fragments and multiple digestions allowed construction of a composite restriction map. The KpnI fragments of pLM2001 were cloned into the KpnI cleavage site of the vector plasmid pDB101. A recombinant plasmid (pSH3) obtained from a lactose-fermenting, erythromycin-resistant (Lac+ Eryr) transformant of Streptococcus sanguis Challis was analyzed by enzyme digestion and agarose gel electrophoresis. Plasmid pSH3 contained 7 of the 11 KpnI-HindIII fragments from pLM2001 and 5 of the 7 fragments from pDB101. It was determined that a 23-kilobase (kb) KpnI-generated fragment from pLM2001 had been cloned into pDB101 with deletion of part of the vector plasmid. The recombinant plasmid could be transformed with high frequency into several Lac- strains of S. sanguis, conferring the ability to ferment lactose and erythromycin resistance. The presence of pSH3 allowed a strain deficient in Enzyme IIlac, Factor IIIlac, and phospho-beta-galactosidase of the lactose phosphoenolpyruvate-dependent phosphotransferase system to efficiently ferment lactose. Under conditions designed to maximize curing of plasmid DNA with acriflavin, no Lac- derivatives could be isolated from cells transformed with pSH3. Seven of the 40 Lac+ colonies isolated after 10 transfers in acriflavin were shown to be sensitive to erythromycin and did not appear to harbor plasmid DNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Transfer of recombinant plasmids containing the gene for DpnII DNA methylase into strains of Streptococcus pneumoniae that produce DpnI or DpnII restriction endonucleases

Plasmid transfer via the transformation pathway of Streptococcus pneumoniae was weakly restricted by the DpnI or DpnII restriction endonuclease, either of which gave a reduction only to 0.4, compared with phage infection, which was restricted to 10(-5). The greater sensitivity of plasmid transfer compared with chromosomal transformation, which was not at all restricted, can be attributed to partially double-stranded intermediates formed from two complementary donor fragments. However, clustering of potential restriction sites in the plasmids increased the probability of escape from restriction. The recombinant plasmid pMP10 , in which the gene for the DpnII DNA methylase was cloned, can be transferred to strains that contain neither restriction enzyme or that contain DpnII as readily as can the vector pMP5 . Introduction of pMP10 raised the level of methylase by five times the level normally present in DpnII strains. Transfer of pMP10 to DpnI -containing strains was infrequent, presumably owing to the suicidal methylation of DNA which rendered it susceptible to the host endonuclease. The few clones in which pMP10 was established had lost DpnI . Loss of the plasmid after curing of the cell eliminated the methylase but did not restore DpnI . Although this loss of DpnI could result from spontaneous mutation, its relatively high frequency, 0.1% suggested that the loss was due to a regulatory shift.

Transformation in Bacillus subtilis: a 75,000-dalton protein complex is involved in binding and entry of donor DNA

A 75,000-dalton protein complex involved in DNA binding during transformation was purified from membranes of competent Bacillus subtilis cells. Previous results (Smith et al., J. Bacteriol. 156:101-108, 1983) showed that the complex contained two polypeptides, polypeptide a (molecular weight, 18,000; isoelectric point, 5.0) and polypeptide b (molecular weight, 17,000; isoelectric point, 4.7) in approximately equal amounts. In the present experiments the two polypeptides were extracted from two-dimensional gels and studied separately and in combination with respect to DNA binding and nuclease activities. For DNA binding the interaction of both polypeptides was required. DNA binding occurred efficiently in the presence of EDTA. Nuclease activity was restricted to polypeptide b. The nucleolytic properties of b were identical to those of the native 75,000-dalton complex. Polypeptide a affected b by reducing its nuclease activity. Analysis of the nuclease subunit b on DNA-containing polyacrylamide gels revealed nuclease activities at four different molecular weight positions. These activities were identical to the major competence-specific nuclease activities which were previously implicated in the entry of donor DNA during transformation (Mulder and Venema, J. Bacteriol. 152:166-174, 1982). These results indicate that the 75,000-dalton protein complex is composed of two different competence-specific polypeptides involved in both binding and entry of donor DNA. The possible roles of the two polypeptides in the transformation of B. subtilis are discussed.

Length of foreign DNA in chimeric plasmids determines the efficiency of its integration into the chromosome of the cyanobacterium Synechococcus R2

The photosynthetic cyanobacterium Synechococcus R2 is efficiently transformed by DNA molecules that contain antibiotic genes from Escherichia coli linked to a segment of Synechococcus R2 chromosomal DNA. Antibiotic-resistant transformants result from integration of donor DNA into the cyanobacterial chromosome by homologous recombination. Foreign DNA interrupting the cyanobacterial sequence in the donor molecule integrates by replacement of homologous recipient DNA with donor DNA containing the foreign insert. Foreign DNA linked to the ends of the cyanobacterial fragment in a circular donor molecule integrates by a reciprocal cross-over between donor and recipient sequences. Using donor molecules that contain different lengths of foreign DNA in both of the above positions, we have determined that the probability of integration decreases by half for each increase of 2 to 3 kb in length of a foreign segment, regardless of its position in the donor molecule. The length of one of the two foreign segments has no effect on the integration of the other. Foreign DNA 20 kb in length is completely stable when it has integrated by the replacement mechanism. The ability to stably introduce large pieces of foreign DNA makes Synechococcus R2 an attractive organism in which to study and modify both native and heterologous genes involved in oxygenic photosynthesis.

Protease-sensitive transfection of Streptococcus pneumoniae with bacteriophage Cp-1 DNA

The transfecting activity of pneumococcal phage Cp-1 DNA was destroyed by treatment with proteolytic enzymes, although these enzymes did not affect transfection with bacteriophage Dp-4 DNA. This transfection was stimulated by calcium ions. Protease-treated Cp-1 DNA competes for binding and uptake with transforming pneumococcal DNA as well as with transfecting Dp-4 DNA to approximately the same extent as does untreated Cp-1 DNA. In addition, [3H]thymidine-labeled Cp-1 DNA, treated with proteases or untreated, was absorbed with the same efficiency. These data suggest that uptake of Cp-1 DNA is not affected by protease treatment. [3H]thymidine-labeled Cp-1 DNA showed remarkable resistance against surface nuclease activity of competent wild-type cells. The monomeric form of the Cp-1 DNA-protein complex showed a linear dose response in transfection.

Transformation in Bacillus subtilis: purification and partial characterization of a membrane-bound DNA-binding protein

In DNA binding-deficient mutants of Bacillus subtilis a competence-specific protein with a subunit molecular weight of 18,000 was absent. The native protein containing this subunit was purified from B. subtilis membranes by chromatography on hydroxyapatite, DEAE-cellulose, and Sephacryl S-200. This protein appeared to be complexed with a second protein of slightly lower molecular weight (17,000) and a different isoelectric point. The native protein complex (apparent molecular weight, 75,000) contained approximately equal amounts of the two polypeptides and showed a strong DNA-binding activity. Incubation of the complex with plasmid and bacteriophage DNA revealed nuclease activity, specifically directed toward double-stranded DNA. Predominantly single-stranded nicks and a limited number of double-stranded breaks were introduced in the presence of Mg2+ ions. In the presence of Mn2+ ions the complex produced low-molecular-weight breakdown products from the DNA.

Helper plasmid cloning in Streptococcus sanguis: cloning of a tetracycline resistance determinant from the Streptococcus mutans chromosome

A model system for testing the helper plasmid cloning system of Gryczan et al. (Mol. Gen. Genet. 177:459-467, 1980) was devised for the Streptococcus sanguis (Challis) host-vector system. In this system, linearized pVA736 plasmid efficiently transformed an S. sanguis (Challis) host containing a homologous plasmid, pVA380-1, but did not transform a plasmidless host or a host containing a nonhomologous plasmid, pVA380. In addition, whereas monomeric circular pVA736 transformed a plasmidless host with two-hit kinetics, it transformed a pVA380-1-containing host with one-hit kinetics. This helper plasmid cloning system was used to isolate two HindIII fragments (5.0 megadaltons [Mdal] and 1.9 Mdal in size) from the chromosome of Streptococcus mutans V825 which conferred high-level tetracycline resistance. One tetracycline-resistant clone was examined and found to contain three plasmids which were sized and designated pVA868 (9.0 Mdal), pVA869 (9.5 Mdal), and pVA870 (9.8 Mdal). Results of Southern blot hybridization and restriction endonuclease digestion confirmed that all three chimeras were composed of two HindIII fragments of the S. mutans V825 chromosome, as well as a large portion, varying in size for each chimera, of the 2.8 Mdal cloning vector, pVA380-1. Incompatibility observed between pVA380-1 and each of the chimeras indicated that replication of the chimeras was governed by the pVA380-1 replicative origin. Southern blotting experiments revealed that the chimeras hybridized to Tn916, providing the first evidence that transposon-related genes of enteric streptococcal origin are disseminated among oral streptococci.

Transformation of Streptococcus lactis Protoplasts by Plasmid DNA

Polyethylene glycol-treated protoplasts prepared from Streptococcus lactis LM3302, a lactose-negative (Lac − ) derivative of S. lactis ML3, were transformed to lactose-fermenting ability by a transductionally shortened plasmid (pLM2103) coding for lactose utilization.

Facilitation of plasmid transfer in Streptococcus pneumoniae by chromosomal homology

The frequency of plasmid establishment in the transformation of Streptococcus pneumoniae by plasmid DNA was increased more than 10-fold when the plasmid carried DNA homologous to the host chromosome. Perfect homology was not necessary for such facilitation; small additions or deletions were tolerated, but extensive deletions in the homologous segment of either plasmid or chromosome reduced or eliminated facilitation. The facilitated plasmid transfer showed a linear dependence on monomeric plasmid concentration rather than the quadratic dependence found in the absence of homology, which indicated that entering plasmid fragments interacted with the chromosome rather than with each other to establish a plasmid replicon. Restriction enzyme cleavage of the plasmid in the nonhomologous segment destroyed its activity, but cleavage in the homologous segment or even enzymatic removal of part of that segment did not prevent plasmid transfer, and plasmids of the original size were established. In facilitated transfer, chromosomal markers (additions and deletions as well as single-site mutations) entered the plasmid with a frequency ranging from 10 to 90% depending on the marker location. Several possible mechanisms for the establishment of plasmids in the presence of chromosomal homology and for the transfer of chromosomal information are considered. They depend on synapsis of the newly entered single-strand plasmid fragment with the host chromosome and subsequent copying of, donation from, or integration into the homologous chromosomal segment. After plasmid establishment, equilibration of donor and chromosomal markers between the chromosome and the plasmid pool, presumably by homologous recombination events, was observed.

Plasmids, drug resistance, and gene transfer in the genus Streptococcus

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Plasmid transformation of Streptococcus sanguis (Challis) occurs by circular and linear molecules

Transformation of Streptococcus sanguis (Challis) by antibiotic resistance plasmids has shown that (a) competence developed with identical kinetics for chromosomal and plasmid DNA; (b) dependence of transformant yield on plasmid DNA concentration was second order; (c) open circular plasmid DNA transformed Challis, although at reduced frequency, (d) linearization of plasmid DNA by restriction enzymes cutting at unique sites inactivated the transforming capacity; (e) transforming activity was restored when linear plasmid molecules generated by different restriction enzymes were mixed; (f) restoration of transforming activity depended on the distance between the linearizing cuts, i.e. on the presence of sufficiently long overlapping homologous sequences; (g) when linear deletion mutants were mixed with linear parental plasmids the smaller plasmid was restored with significantly higher frequency. Based on these data, a model for plasmid transformation of Challis is proposed according to which circular plasmid is linearized during binding and uptake. One DNA strand enters the cell and restoration of circular plasmids inside the cell occurs by annealing of complementary single strands from two different donor molecules. Implications of this model for recombinant DNA experiments in streptococci are discussed.

Plasmid transformation in Bacillus subtilis: fate of plasmid DNA

Only multimeric, and not monomeric forms of B. subtilis plasmids can transform B. subtilis cells (Canosi et al. 1978). This finding prompted us to study the physico-chemical fate of plasmid DNA in transformation. Competent cells of B. subtilis were exposed to either unfractionated preparations or to preparations of multimeric plasmid DNA. Plasmid DNA was re-extracted from such cells and then analyzed by sedimentation and isopycnic centrifugation and also defined by its sensitivity to nuclease S1 degradation. No double-stranded plasmid DNA could be recovered from cells transformed with unfractionated plasmid preparations which contained predominantly monomeric covalently closed circular (CCC) DNA. Re-extracted plasmid DNA was single-stranded, had a molecular weight considerably smaller than monomer length DNA and had been subject to degradation to acid soluble products. However, when transformations were performed with multimeric DNA (constructed by in vitro ligation of linearized pC194 DNA), both double-stranded and partially double-stranded DNA could be recovered in addition to single-stranded DNA. We assume that plasmid DNA is converted to a single-stranded form in transformation, irrespective of its molecular structure. Double-stranded and partially double-stranded DNAs found in transformation with multimeric DNA would be the products of intramolecular annealing.

Pathway of plasmid transformation in Pneumococcus: open circular and linear molecules are active

We have extended the analysis of plasmid transformation in Streptococcus pneumoniae by finding that monomeric and dimeric open circular and linear forms of pMV158 were active in transformation. Their efficiencies were at least 35-fold lower than those of the corresponding closed circular forms. The evidence came largely from analysis of S1 nuclease-digested plasmid deoxyribonucleic acid by combinations of dye-buoyancy, gel electrophoresis, and sedimentation velocity methods. As with closed circular forms, monomer open circular forms gave second-order kinetics and dimer forms gave first-order kinetics. Unique linear products of digestion by either of two restriction enzymes were inactive, but a mixture of the two digests was active, as was the mixture of linear monomer deoxyribonucleic acids produced by S1 nuclease. Absolute efficiencies of transformation were low even for closed circular donors. All of the results, including the low efficiencies, were consistent with the interpretation that plasmid replicons were assembled in the recipient cell by pairing of fragments of single strands that had entered the cell separately from duplex donors that had been cut on the cell surface.

Uptake of plasmid deoxyribonucleic acid by Haemophilus

The uptake of circular and linear plasmid RSF0885 deoxyribonucleic acids, (DNAs) obtained from Haemophilus parainfluenzae 14, in both homologous and heterologous recipients was studied and compared with that of chromosomal DNA. High concentrations of divalent cations stimulated the uptake of either circular or linear plasmid DNA in H. parainfluenzae 14 competent cells but did not affect the uptake of chromosomal DNA. The biological activity of linear plasmid DNA was similar to that of circular DNA, and the transforming efficiencies for ampicillin resistance of both molecular forms were stimulated by divalent ions. Plasmid DNA was taken up efficiently either with or without the addition of divalent ions but was not biologically active in the heterologous Haemophilus influenzae Rd recipient. Our results suggest that in H. parainfluenzae 14 some of the steps for chromosomal and plasmid DNA uptake are different.

Cloning of Streptococcus pneumoniae DNA: its use in pneumococcal transformation and in studies of mismatch repair

EcoRI fragments of the amiA locus in Streptococcus pneumoniae were cloned either into a derivative of lambda or into pBR325 plasmid. Mutations in the amiA locus confer resistance to aminopterin. Pneumococcal DNA fractions were enriched for the desired EcoRI fragments by agarose gel electrophoresis. Recombinant clones were detected directly by transformation with DNA and lambda plaques or from single-colony lysates containing pBR325. The use of cloned DNA in pneumococcal transformation has revealed a number of features pertinent to transformation in general, and also the mismatch repair process. High transformation levels can be achieved, from 40 to 80% of a competent culture. These high levels of transformation with cloned DNA made in a foreign host are taken to confirm the absence of restriction effects on transformation in S. pneumoniae. At saturation, similar transformation levels are obtained with hybrid phage or hybrid plasmid DNAs, but the DNA amount required is 20 to 25 times lower for hybrid plasmid than for hybrid phage, probably because plasmid DNA is 10 times shorter than phage DNA. There is no "end effect" with intact hybrid DNA, i.e. similar transformation levels are achieved for markers whatever their map position on the cloned pneumococcal fragment. Cloned DNA has been used to study the action of the mismatch repair process (hex system). The presence of two mismatches in the same cell is not enough to saturate the hex system, and is not enough to kill the colony-forming center (cfc).

Monomer plasmid DNA transforms Streptococcus pneumoniae

The covalently closed (CC) monomer form of plasmid pMV158 was found to transform pneumococcus (Streptococcus pneumoniae) and to do so with two-hit kinetics. The evidence came from analysis of the behavior of the transforming activity in fractions from preparative gel electrophoresis. Activity in the first major peak to elute (i) co-eluted with monomer CC as detected on analytical gels, (ii) banded as CC in dye-buoyancy gradients, (iii) sedimented with the velocity expected for monomer CC, and (iv) gave two-hit kinetics as functions of both concentration and time of exposure of the cells to DNA. A second major peak of activity behaved physically as though mostly due to dimer CC forms and gave single-hit response curves. Because almost no dimer was detectable optically on analytical gels of starting preparations, its specific activity was high relative to that of the monomers.

Genetic transformation of Streptococcus pneumoniae by heterologous plasmid deoxyribonucleic acid

A number of heterologous plasmid deoxyribonucleic acids (DNAs) coding for erythromycin, tylosin, lincomycin, tetracycline, or chloramphenicol resistance have been introduced into Streptococcus pneumoniae via genetic transformation with frequencies that varied between 10(-5) to as high as 5 x 10(-1) per colony-forming unit. Transformation with plasmid DNA required pneumococcal competence, was competed by chromosomal DNA, and showed a saturation at about 0.5 micrograms/ml (with a recipient population of 3 x 10(7) colony-forming units of competent cells per ml). Plasmid transformation did not occur with a recipient strain, 410, defective in endonuclease I activity and in chromosomal genetic transformation. All erythromycin-resistant transformants examined contained covalently closed circular DNA with the same electrophoretic mobility on agarose gels as the donor DNAs, and when examined in detail the plasmid reisolated from the transformants had the same restriction patterns and the same specific transforming activity as the donor DNA. In the cases of two plasmids examined in detail--pAM77 and pSA5700 Lc9--most of the transforming activity was associated with DNA monomers; DNA multimers present in pSA5700 Lc9 also had biological activity. An unexpected finding was the demonstration of transformation (2 x 10(-5) per colony-forming unit) with plasmid DNAs linearized by treatment with S1 nuclease or with restriction endonucleases.

Interactions of competent Streptococcus sanguis (Wicky) cells with native or denatured, homologous or heterologous deoxyribonucleic acids

Competent cell-deoxyribonucleic acid (DNA) interactions were examined using tritium-labeled homologous or heterologous native or denatured DNAs and competent Streptococcus sanguis Wicky cells (strain WE4). The DNAs used were extracted from WE4 cells, Escherichia coli B cells, and E. coli bacteriophages T2, T4, T6, and T7. The reactions examined were: (i) total DNA binding, (ii) deoxyribonuclease-resistant DNA binding, and (iii) the production of acid-soluble products from the DNA. Optimal temperatures for the reactions were as follows: reaction (i), between 30 and 40 degrees C; reaction (ii), 30 degrees C; and reaction (iii), greater than 40 degrees C. The rates for the reactions (expressed as molecules of DNA that reacted per minute per colony-forming unit) did not vary greatly from one DNA source to another. With a constant competent cell concentration and differing DNA concentrations below a saturation level (from a given source), a different but constant fraction of the added DNA was cell bound, deoxyribonuclease resistant, and degraded to acid-soluble products. In experiments where the number of competent cells was varied and the DNA concentration was held constant, again essentially the same result was obtained. The extent of reactions (i), (ii), and (iii) depended upon the numbers as well as the source of DNA molecules applied to competent cells. Calcium ion essential for native DNA-cell reactions was also found essential for denatured DNA-cell reactions. Data obtained from competition experiments lead to the conclusion that competent WE4 cells contain specific sites for native as well as denatured DNAs.

Polyethylene glycol-assisted transfection of Streptomyces protoplasts

In the presence of polyethylene glycol (concentration optimum 20%), protoplasts of appropriate Streptomyces strains could be transfected by deoxyribonucleic acid (DNA) of five temperate phages (phi C31, VP5, R4, phi 448, and S14) belonging to four different immunity groups. Quantitation of transfection was made possible by plating the transfection mixture with excess uninfected protoplasts in soft agar overlays on protoplast regeneration medium so that plaques were easily detected. Optimum frequencies of transfection in the ranges of 10(-6)/DNA molecule and 10(-5)/viable protoplast were invariably obtained. It appeared that single DNA molecules initiated transfection events, and that the conformation of the DNA (i.e., circular or linear) was not important. Inhibition of transfection by ethylenediaminetetraacetic acid suggested that divalent cations were also observed. A minor subpopulation of protoplasts appeared to be particularly sensitive to transfection (i.e., "competent") in some DNA-host combinations. In such cases the size of this subpopulation was the major limiting factor in obtaining high transfection frequencies. The same protoplast

Molecular cloning of heterologous chromosomal DNA by recombination between a plasmid vector and a homologous resident plasmid in Bacillus subtilis

The difficulty experienced in the shotgun cloning of chromosomal DNA on plasmid vectors in Bacillus subtilis is analyzed and an explanation for this difficulty is offered based on an inherent property of competent cells which imposes a requirement of plasmid multimers in transformation of plasmid-free recipients (Canosi et al., 1978). A stratagem which uses cloning by recombination between the vector and a resident homologous plasmid is tested and shown to be successful. Several recombinant plasmids are obtained containing Bacillus licheniformis DNA fragments which complement aromatic amino acid mutants of Bacillus subtilis. The yield of recombinant clones ranges from 6.7 to 210 per microgram of chromosomal DNA, depending on the selection and the restriction endonuclease. The various trp clones obtained after cutting chromosomal DNA with BglII and BclI do not complement trpE and exhibit both orientations with respect to the vector. The location of several restriction endonuclease cleavage sites in the cloned trp fragments is presented, and their relationship to the genetic map of Bacillus licheniformis is described.