RESUMO
BACKGROUND: Natural transformation enables acquisition of adaptive traits and drives genome evolution in prokaryotes. Yet, the selective forces responsible for the evolution and maintenance of natural transformation remain elusive since taken-up DNA has also been hypothesized to provide benefits such as nutrients or templates for DNA repair to individual cells. RESULTS: We investigated the immediate effects of DNA uptake and recombination on the naturally competent bacterium Acinetobacter baylyi in both benign and genotoxic conditions. In head-to-head competition experiments between DNA uptake-proficient and -deficient strains, we observed a fitness benefit of DNA uptake independent of UV stress. This benefit was found with both homologous and heterologous DNA and was independent of recombination. Recombination with taken-up DNA reduced survival of transformed cells with increasing levels of UV-stress through interference with nucleotide excision repair, suggesting that DNA strand breaks occur during recombination attempts with taken-up DNA. Consistent with this, we show that absence of RecBCD and RecFOR recombinational DNA repair pathways strongly decrease natural transformation. CONCLUSIONS: Our data show a physiological benefit of DNA uptake unrelated to recombination. In contrast, recombination during transformation is a strand break inducing process that represents a previously unrecognized cost of natural transformation.
Assuntos
Acinetobacter/genética , Acinetobacter/efeitos da radiação , Evolução Biológica , Análise Custo-Benefício , Transformação Bacteriana/genética , Transformação Bacteriana/efeitos da radiação , Acinetobacter/enzimologia , Acinetobacter/metabolismo , Proteínas de Bactérias/genética , Proteínas de Bactérias/metabolismo , Proteínas de Bactérias/efeitos da radiação , Dano ao DNA/efeitos da radiação , Reparo do DNA/fisiologia , Reparo do DNA/efeitos da radiação , DNA Bacteriano/genética , DNA Bacteriano/efeitos da radiação , Exodesoxirribonuclease V/metabolismo , Exodesoxirribonuclease V/efeitos da radiação , Deleção de Genes , Transferência Genética Horizontal/genética , Transferência Genética Horizontal/efeitos da radiação , Genes Bacterianos/genética , Genes Bacterianos/efeitos da radiação , Proteínas de Membrana/genética , Proteínas de Membrana/efeitos da radiação , Mutação/genética , Mutação/efeitos da radiação , Fenótipo , Recombinação Genética/efeitos da radiação , Estresse Fisiológico , Sobrevida , Raios Ultravioleta/efeitos adversosRESUMO
Class-1 integrons play an important role in the emergence and spread of antimicrobial resistance determinants. In a recent study we showed that host fitness was dramatically reduced following acquisition of these elements. These fitness costs were due to the presence of an active integrase and we suggested that the mechanistic explanation was due to reduced genetic stability through IntI1 mediated recombination events between attI/attC and non-canonical sites in the chromosome. Here we demonstrate that the attI degenerated target sequence is highly prevalent in our model organism Acinetobacter baylyi adding support to the hypothesis that IntI1 is costly due to genomic instability.
RESUMO
Horizontal gene transfer (HGT) plays a major role in bacterial microevolution as evident from the rapid emergence and spread of antimicrobial drug resistance. Few studies have however addressed the population dynamics of newly imported genetic elements after HGT. Here, we show that newly acquired class-1 integrons from Salmonella enterica serovar Typhimurium and Acinetobacter baumannii, free of associated transposable elements, strongly reduce host fitness in Acinetobacter baylyi. Insertional inactivation of the integron intI1 restored fitness, demonstrating that the observed fitness costs were due to the presence of an active integrase. The biological cost of harboring class-1 integrons was rapidly reduced during serial transfers due to intI1 frameshift mutations leading to inactivated integrases. We use a mathematical model to explore the conditions where integrons with functional integrases are maintained and conclude that environmental fluctuations and episodic selection is necessary for the maintenance of functional integrases. Taken together, the presented data suggest a trade-off between the ability to capture gene cassettes and long-term stability of integrons and provide an explanation for the frequent observation of inactive integron-integrases in bacterial populations.
Assuntos
Acinetobacter baumannii/enzimologia , Proteínas de Bactérias/metabolismo , Instabilidade Genômica/fisiologia , Integrases/metabolismo , Integrons/fisiologia , Salmonella typhimurium/enzimologia , Acinetobacter baumannii/genética , Proteínas de Bactérias/genética , Sequência de Bases , Integrases/genética , Dados de Sequência Molecular , Salmonella typhimurium/genéticaRESUMO
A straightforward method for the synthesis of virtually enantiomerically pure ruthenium(II) polypyridyl complexes [Ru(pp)(pp')(pp")](PF(6))(2), pp = bidentate polypyridyl has been developed. The synthesis draws from the readily available racemic starting material cis-[Ru(pp)(pp')Cl(2)] and the natural amino acids l- or d-proline and relies on a dynamic asymmetric transformation under thermodynamic control.