Genetic recombination of Xenopus laevis 5 S DNA in bacteria

The behavior in genetic recombination of Xenopus laevis 5 S DNA has been examined, with particular emphasis on the role of 15-base-pair tandem repeats in the A + T-rich spacer. Fragments of 5 S DNA were introduced into Escherichia coli cells as inserts in the recombination vectors, lambda rva and lambda rvb. Intermolecular recombinants were selected in which, because of properties of the phage vectors, the crossover event must have occurred within the 5 S DNA inserts. Inserts from individual recombinants have been characterized in detail. The effects of varying the number (n) of 15-base-pair repeats and the recombination capabilities of the phage and host have been investigated. In these crosses, unequal crossovers can occur, yielding inserts different in size from the parental inserts. When the number of 15-mers is large (n = 12 or 20), most of the unequal crossovers have occurred within the 15-mers, resulting in an altered n value, although other homologies within the 5 S DNA sequence can also support unequal events. Increasing n in the parental inserts modestly increases the overall frequency of recombination and the percentage of altered inserts. We conclude that, in a bacterial setting, the 15-base-pair repeats stimulate recombination only slightly by allowing alternative registers for heteroduplex formation. The degree of stimulation observed is less than predicted by one simple model.