Expression of genes int and xis was measured in ColE1 hybrid plasmids carrying the att-int-xis genes on EcoRI segment C of λ DNA. The requirements for synthesis of λ integration and excision functions by these hybrid plasmids are similar to those previously observed for intact phage. Since the plasmids lack the pl promoter, no xis product is made, but enough int product is produced constitutively to catalyze low but significant excisive recombination, if xis product is supplied. To obtain enough int function for integrative recombination, the cII and cIII products must be supplied. Hybrid plasmids carrying a similar fragment from phages λint-c (either a point mutation or an IS2 insertion in gene xis) express high levels of int function constitutively and promote integrative recombination even in the absence of cII and cIII products. They also promote low but significant levels of excisive recombination even in the absence of xis function. For both λ and λint-c fragments, int expression appears to depend on promoters within the fragment, but not on plasmid promoters, since such expression is independent from the orientation of the C fragment in the plasmid.
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