A novel assay for illegitimate recombination in Escherichia coli: stimulation of lambda bio transducing phage formation by ultra-violet light and its independence from RecA function.

We developed a novel assay system for illegitimate recombination, in which the frequency of the formation of lambda Spi- phages formed during prophage induction was measured with an E. coli P2 lysogen as the indicator bacteria. Since almost all of the lambda Spi- phages thus detected contain attR, they have essentially the same structures as lambda bio transducing phages, indicating that this assay system enables us to detect specialized transducing phages that produce heterogenote transductants, thus ignoring the occurrences of docL and docR particles which carry only one cohesive end. The following results on the formation of specialized transducing phages have been obtained by this assay system to date. (1) Irradiation with UV light greatly enhanced the formation of lambda Spi- phages. (2) Treatments with other DNA-damaging agents also enhanced the formation of lambda Spi- phages. (3) Illegitimate recombination during prophage induction does not require the RecA function, indicating that enhancement of lambda Spi- phage formation is not controlled by the SOS regulatory system. (4) Preliminary results suggested that DNA gyrase is involved in the formation of lambda Spi- phage during pro-phage induction. Since the above results were consistent with most of the previous observations on the illegitimate recombination in other systems, the Spi- assay system can provide important clues to the mechanism of illegitimate recombination.