Topography and kinetics of genetic recombination in Escherichia coli treated with psoralen and light.

Genetic exchanges appear to be involved in repair of cross-linked DNA. Kinetics for completion of repair and strand rejoining controlled by the recA(+) gene were examined in Escherichia coli treated with psoralen and light. The results suggest the following model for genetic recombination. After cross-linking treatment, cells in a population initiate repair in near synchrony. Removal of DNA cross-links, preparation of substrate for recombination, and initiation of the first recA-dependent event are completed in less than 1 min. Recombination events occur singly in each cell or chromosome, and require 2.3 +/- 0.4 min at 32 degrees for the recA(+)-dependent step. After completion of the first event, subsequent recombination events occur in a sequential or progressive fashion around the chromosome or in clusters which may consist of one or more domains of the folded chromosome. The time required to proceed to successive sites is either a constant, independent of the distance on the chromosome, or is quite small compared to 2.3 min. DNA substrate for recombination decays with approximate first-order kinetics and the rate is dependent on the number of unrepaired sites. Cell survival can be expressed as a competition between completion of all repair events and the simultaneous decay of chromosomes to forms not reparable by recombination.Equations relating kinetics for completion of repair, the size distribution of DNA molecules, and cell survival are derived for the above model, using as parameters only rate constants for recombination and decay of substrate, and number of events per chromosome. An excellent correlation is found between experimentally determined and theoretical values.