DEFINING THE STRUCTURAL AND FUNCTIONAL ROLES OF THE CARBOXYL REGION OF THE BACTERIOPHAGE-LAMBDA EXCISIONASE (XIS) PROTEIN

The bacteriophage lambda excisionase (Xis) protein is required for exc isive site-specific recombination. Xis is composed of 72 amino acids a nd binds cooperatively to two DNA sites (X1 and X2) that are arranged as direct repeats. Alternatively, Xis binds cooperatively with the hos t-encoded factor for inversion stimulation (FIS) protein at the X1 and F sites, respectively. Here we analyzed the effects of missense subst itutions from codon 57 to the carboxyl end of the protein and nonsense mutations that truncate the protein at various positions from residue s 60 to 69. We find that all of the mutant proteins promote excision t o some extent and interact cooperatively with FIS. Some mutants have n o detectible phenotype while others are altered in their abilities to promote excision or to interact cooperatively with integrase (Int). Co mputer modeling predicts that amino acids from residues 59 to 65 are i n an alpha-helix conformation. Mutants with substitutions on one side of the helix at residues 57, 60, 63 and 64 as well as truncated mutant s containing 60, 61 or 63 amino acids, fail to interact cooperatively with Int suggesting that this region of the protein forms the interfac e with Int. Mutants with substitutions at other positions in the putat ive helix have no detectible phenotype. Residues 66 to 68 may form a r everse turn and the last four amino acids (69 to 72) may not be crucia l for the structure or function of the protein. (C) 1998 Academic Pres s.