The helicase domain of phage P4 alpha protein overlaps the specific DNA binding domain

The ATPase and helicase activities of gene product alpha of phage 4 of Escherichia coli in a series of mutants that are modified in the NTP binding motif or at the termini of the protein were examined to define the helicase domain. Mutations of residues G506 and K507 abolish both NTPase and helicase activities and are defective in unwinding a forked helicase substrate, thus suggesting that the proposed P loop is essential for helicase activity. A series of truncations constructed at the 5' and 3' ends of the alpha gene showed that 142 residues of the C terminus are sufficient for the specific binding of ori and crr DNA. The C terminus was also found to be required for helicase activity and for specific DNA binding.