Molecular Genetics

A site-specific recombination system acts in replication of bacterial chromosomes.

Facts

Odd numbers of recombination events between replicated portions of Cairn's replication intermediates inevitably lead to concatemer formation without a deconcatenation mechanism. Concatemer formation and resolution are best studied in plasmids, such as the Col E1 plasmid in E. coli. Failure to accumulate concatemers occurs with plasmids whose cer locus is mutated and in E.coli strains with mutations in the xer locus. The cer locus acts in cis and the xer loci can act in trans.

The XerC and the XerD proteins are each site-specific recombinases of the Int-Flp family. The cer locus contains two half sites with different binding specificities, one for XerC and one for XerD, as determined by footprinting and mobility shift analysis.

• The only orientations of half sites that are active in recombination are those in which both half sites on one side of the exchange bind XerC and those on the other side bind XerD.
• Separately, XerC and XerD each have topoisomerase I activity (Ref).

Interpretations

• Replicating circular genomes require proteins that resolve topological tangles.
• Asymmetry of the recombinase binding sites guarantees that only the resolving reaction takes place in this case, whereas with Flp, it is asymmetry of the spacer sequence that is important.
• Many topological problems are avoided by linear chromosomes.

Further information

• XerC acts before XerD creating a Holliday intermediate.
• The E. coli chromosome contains a different target site for concatemer resolution.
• Replication without recombination or with an even number of recombination events produces catenanes. These are resolved by topoisomerase II activities.

This is page 3213 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2000