Escherichia coli strains containing mutations in the structural gene for topoisomerase I are recombination deficient.

Mutations in the gene encoding topoisomerase I of Escherichia coli were tested for their effect on plasmid recombination. Recombination was decreased 1,000-fold at 30 and 37 degrees C and occurred at approximately wild-type frequencies at 42 degrees C. The suppression of topA mutations at 42 degrees C did not appear to be a result of increased topoisomerase I activity at 42 degrees C.