Properties of the int and xis Gene Products
How many phage-encoded proteins are required for integration and excision? If Int and Xis proteins will function in trans, then complemen-tation tests can be performed to determine the number of int genes and the number of xis genes carried on the phage. Experiments showed that the Int protein of one phage will indeed help another phage integrate, for cells coinfected with a λint mutant and a heteroimmune λimm434 phage yield some lambda lysogens in addition to λimm434 lysogens and
double lysogens. In contrast, coinfection of cells with two different λint mutants does not yield lysogens at the frequencies normally observed for complementation. All the int mutations tested therefore must lie in
the same gene. Analogous experiments with xis mutants show that they too are confined to a single gene.
Int and xis mutations are not complemented by most phage otherthan lambda. Although φ21 possesses its own Int and Xis proteins, these do not complement lambda int or xis mutants. Thus, int and xis can be phage-specific.