MUTATIONS ABOLISHING THE ENDONUCLEASE ACTIVITY OF BACTERIOPHAGE-LAMBDA TERMINASE LIE IN 2 DISTINCT REGIONS OF THE A-GENE, ONE OF WHICH MAY ENCODE A LEUCINE ZIPPER DNA-BINDING DOMAIN

Bacteriophage λ terminase is a multifunctional enzyme composed of two subunits which are the products of the phage-encoded Nu1 and A genes. The enzyme catalyzes the endonucleolytic cleavage of λ DNA at a site known as cosN and mediates packaging of the phage DNA into empty heads. This work describes the characterization of mutations within the A gene which lead to the loss of terminase endonuclease activity without affecting the ability of the enzyme to package monomeric mature (cut) λ DNA. The residues changed by these mutations lie in two distinct regions within the carboxy half of the A protein. One of these regions has sequence homology with a conserved region of DNA polymerases. The other region resembles the "leucine zipper" DNA binding domain (bZIP) found in eukaryotic transcription factors in that both a basic region and leucine heptad-repeat are present. This terminase domain may be involved in the recognition and/or cleavage of cosN. © 1992.