Interspecies genetic exchange is an important evolutionary mechanism in bacteria. It allows rapid acquisition of novel functions by transmission of adaptive genes between related species. However, the frequency of homologous recombination between bacterial species decreases sharply with the extent of DNA sequence divergence between the donor and the recipient. In Bacillus and Escherichia, this sexual isolation has been shown to be an exponential function of sequence divergence. Here we demonstrate that sexual isolation in transformation between Streptococcus pneumoniae recipient strains and donor DNA from related strains and species follows the described exponential relationship. We show that the Hex mismatch repair system poses a significant barrier to recombination over the entire range of sequence divergence (0.6 to 27%) investigated. Although mismatch repair becomes partially saturated, it is responsible for 34% of the observed sexual isolation. This is greater than the role of mismatch repair in Bacillus but less than that in Escherichia. The remaining non-Hex-mediated barrier to recombination can be provided by a variety of mechanisms. We discuss the possible additional mechanisms of sexual isolation, in view of earlier findings from Bacillus, Escherichia, and Streptococcus.Bacteria from all major taxa are able to exchange genes across species by homologous recombination (26). While the various bacteria take up donor DNA by a diversity of mechanisms, all studied systems of homologous recombination share at least one homologous feature: recombination depends ultimately on the activity of the RecA protein and its homologues (26). Similarly, there is one system that hinders recombination across species in both the Proteobacteria and the gram-positive bacteria (3). This is the mismatch repair system encoded by mutS, mutL, and their homologues. Because some of the molecular basis for interspecies recombination is shared across disparate taxa, we might expect that recombination between species is constrained in similar ways throughout the bacterial world.In addition, previous studies have shown that the frequency of homologous recombination decreases with the sequence divergence between donor and recipient, in a manner that is similar across a wide range of organisms. In Bacillus transformation as well as in Escherichia conjugation, the frequency of recombination decreases exponentially with the degree of DNA sequence divergence between donor and recipient (23,28,31). A similar exponential relationship has been observed for the frequency of intrachromosomal crossovers in Saccharomyces cerevisiae (5). Nevertheless, the major mechanisms producing recombinational barriers have been shown to differ in each of the above cases. In Escherichia coli, the predominant barrier to recombination is presented by the methylation-directed mismatch repair system (21). In Bacillus, mismatch repair is only marginally effective in preventing recombination between divergent sequences; the most significant barrier is that donor stran...
The aim of the present study was to examine the stability and evolution of tet(M)-mediated resistance to tetracyclines among members of different clonal lineages of Streptococcus pneumoniae. Thirty-two tetracyclineresistant isolates representing three national (Spanish serotype 14, Spanish serotype 15, and Polish serotype 23F) and one international (Spanish serotype 23F) multidrug-resistant epidemic clones were all found to be tet(M) positive and tet(O), tet(K), and tet(L) negative. These isolates all carried the integrase gene, int, which is associated with the Tn1545-Tn916 family of conjugative transposons. High-resolution restriction analysis of tet(M) products identified six alleles, tet(M)1 to tet(M)6: tet(M)1 to tet(M)3 and tet(M)5 in isolates of the Spanish serotype 14 clone, tet(M)4 in both the Spanish serotype 15 and 23F clones, and tet(M)6, the most divergent allele, in the Polish 23F clone. This indicates that tet(M) variation can occur at the inter-and intraclone levels in pneumococci. Two alleles of int were identified, with int1 being found in all isolates apart from members of the international Spanish 23F clone, which carried int2. Susceptibility to tetracycline, doxycycline, and minocycline was evaluated for all isolates with or without preincubation in the presence of subinhibitory concentrations of tetracyclines. Resistance to tetracyclines was found to be inducible in isolates of all clones; however, the strongest induction was observed in the Spanish serotype 15 and 23F clones carrying tet(M)4. Tetracycline was found to be the strongest inducer of resistance, and minocycline was found to be the weakest inducer of resistance.The gram-positive pathogen Streptococcus pneumoniae (the pneumococcus) is a major cause of pneumonia, otitis media, and meningitis (12). The evolution and broad global distribution of multiple antibiotic resistance determinants in bacteria have resulted in a situation in which pneumococci are commonly resistant to penicillin, the broad-spectrum cephalosporins, macrolides, lincosamides, co-trimoxazole, chloramphenicol, and tetracyclines, as well as rifampin (11), sulphonamides (42), and fluoroquinolones (13,24,30), making the treatment of serious pneumococcal disease increasingly difficult (17,22). The transformable nature of S. pneumoniae (which has played an important role, along with point mutations) in the evolution of resistance [1,4,11,13,24,30] has in no small part also led to a population structure characterized by free genetic exchange, punctuated by clonal expansion of successful variants. The best studied of these are the Spanish 23F, Spanish 6B, and French/ Spanish 9V14 multidrug-resistant clones that have now spread intercontinentally (see reference 10 for a recent review).One class of antimicrobial agents found most often in clinical use is the tetracyclines, broad-spectrum bacteriostatic drugs shown to be active against pneumococci (33). In some European (9, 16, 23), Asian (35,36,41,47), and African (31, 52), countries lack of susceptibility to tetracyclines...
We have surveyed naturally occurring plasmids in strains of Bacillus subtilis and the closely related species B. mojavensis and B. licheniformis. Previous studies have failed to find host-benefitting functions for plasmids of these species, suggesting that these plasmids are nonmutualistic. Only one type of plasmid was found in each plasmid-bearing strain, suggesting that most of the plasmids infecting these Bacillus species are in the same incompatibility group. A sample of 18 plasmids from these species ranged in size from 6.9 to 16 kb, with all but 6 plasmids falling into three size groups. These groups differed in the sizes of their host ranges and geographical ranges. All but 1 of the 18 plasmids from these three host species are homologous with one another. The cryptic plasmids from these three species are far less diverse than are plasmids (from other species) that are known to benefit their bacterial hosts. The low-level diversity among these cryptic plasmids is consistent with the hypothesis that host-benefitting adaptations play an important role in fostering the coexistence of plasmid populations, but other explanations for the low-level plasmid diversity are possible. Comparison of the phylogenies of the plasmids with those of their hosts suggests that Bacillus plasmids are horizontally transferred in nature at a low rate similar to that found for the colicin plasmids of Escherichia coli.
The relationship between sexual isolation and sequence divergence in Bacillus transformation was previously shown to be log linear. In the present study, we have shown that this relationship is robust with respect to naturally occurring genetic variation among recipient strains of Bacillus subtilis and B. mojavensis. Naturally occurring restriction endonuclease activity was shown not to affect this relationship. Also, seven out of eight recombination mutants tested for their sensitivity to sequence divergence have shown the same relationship between sequence divergence and sexual isolation; a mutant for recH was more sensitive to sequence divergence, suggesting that the product of this gene may be involved in resolution of mismatches in heterogamic transformation. We have also shown that the relationship between sexual isolation and sequence divergence is robust with respect to variation in the conditions of transformation, including variation in the length of donor DNA, the concentration of donor DNA, and intracellular competition between donor-derived and recipient-derived DNA. The robustness of the relationship between sexual isolation and sequence divergence among naturally occurring strains and across transformation conditions allows us to predict the eventual outcome of sequence divergence among B. subtilis and its closest relatives.
We investigated the size and continuity of DNA segments integrated in Bacillus subtilis transformation. We transformed B. subtilis strain 1A2 toward rifampicin resistance (coded by rpoB) with genomic DNA and with a PCR-amplified 3.4-kb segment of the rpoB gene from several donors. Restriction analysis showed that smaller lengths of donor DNA integrated into the chromosome with transformation by PCR-amplified DNA than by genomic DNA. Nevertheless, integration of very short segments (< 2 kb) from large, genomic donor molecules was not a rare event. With PCR-amplified segments as donor DNA, smaller fragments were integrated when there was greater sequence divergence between donor and recipient. There was a large stochastic component to the pattern of recombination. We detected discontinuity in the integration of donor segments within the rpoB gene, probably due to multiple integration events involving a single donor molecule. The transfer of adaptations across Bacillus species may be facilitated by the small sizes of DNA segments integrated in transformation.
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