CROSSES BETWEEN INSERTION AND POINT MUTATIONS IN GENE cI: STIMULATION OF NEIGHBORING RECOMBINATION BY HETEROLOGY

M. Lieb ¹, M. M. Tsai ², and Richard C. Deonier ²

¹ Department of Microbiology, University of Southern California, School of Medicine, Los Angeles, California 90033
² Molecular Biology, Department of Biological Sciences, University of Southern California, Los Angeles, California 90089-1481

Intragenic recombination between cI point mutations and insertions was studied in four-factor crosses. In crosses between two point mutations, there is a linear relationship between recombination frequency and distance. However, in crosses between an insertion and point mutations, there is additional recombination in the regions 200 base pairs to the right and to the left of the insertion. The recombinational stimulation occurred with IS insertions and also with insertions consisting of HindIII fragments of SV40 and with a deletion that removes part of cI. This indicated that the stimulation was a result of heterology per se rather than of information encoded by the insertions. Either Rec or Red functions are sufficient for enhanced recombination near a heterology. The stimulation is attributed to more frequent resolution of recombinational intermediates in the neighborhood of a heterology. "Stalling" of migrating branches or invading strands at a heterology may increase the probability of local DNA cleavage.

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