Genetic Exchange and Homologous Recombination

After Escherichia coli, Bacillus subtilis is genetically the best-characterized bacterium. Homologous recombination has been an essential tool in the genetic analysis of B. subtilis, and in turn the availability of powerful genetic tools has facilitated investigation of the exchange of DNA-encoded information in this organism. This review summarizes the present state of knowledge concerning several modes of homologous genetic exchange and places emphasis on natural competence, about which much is known. A section discusses our understanding of the genetics and biochemistry of homologous recombination per se. Another provides an overview of the essential properties of these systems. The chapter emphasizes the newer work that concerns the regulation of competence and the nature of transformation-specific gene products. Competence is widespread in both gram-positive and gram-negative bacteria. The best-studied systems are those of Streptococcus pneumoniae, Streptococcus sanguis, Haemophilus influenzae, Neisseria gonorrhoeae, and B. subtilis. Several competence loci have been identified following the isolation of commutants. The authors propose that the five small ComG proteins assemble to form part of a cell surface-associated structure for the binding and uptake of transforming DNA. The implications of competence- linked induction of DNA repair genes and a possible mechanism for competence-linked induction of the SOS system are also discussed.