Xer Site‐Specific Recombination: Promoting Chromosome Segregation

Abstract

Xer site-specific recombination was discovered in 1984 through its role in converting multimers of ColE1-related multicopy plasmids to monomers and hence ensuring their stable inheritance within Escherichia coli. Research on Xer recombination has been devoted to: (i) characterizing the recombination reaction at the molecular level; (ii) understanding how the system converts plasmid and chromosomal dimers to monomers (and not monomers to dimers); and (iii) understanding how Xer recombination at chromosomal dif is integrated into other aspects of DNA metabolism and processing during the bacterial cell cycle. This chapter focuses on those features of the Xer system that differentiate it from the related site-specific recombination systems, Cre-loxP,λ Int-attP/attB, and Flp-frt. In cells containing circular chromosomes, chromosome dimers can arise by RecA-dependent homologous recombination between sister chromatids. Studies of Xer site-specific recombination have revealed new mechanistic details of the tyrosine recombinase mechanism. Uncovering the relationships between Xer recombination and chromosome segregation and how these relate to cell division, homologous recombination, and DNA replication is revealing the sophisticated methods that have evolved in bacteria to ensure the stable duplication and inheritance of the genetic material. The characterization of a strategy to ensure that Xer recombination at chromosomal dif is restricted to converting chromosome dimers present at cell division to monomers reveals one new way in which bacteria can process temporal and spatial information.