Genetics, Vol 119, 477-484, Copyright © 1988

INVESTIGATIONS

Domains for Protein-Protein Interactions at the N and C Termini of the Large Subunit of Bacteriophage {lambda} Terminase

W. F. Wu, S. Christiansen and M. Feiss
Department of Microbiology, College of Medicine, University of Iowa, Iowa City, Iowa 52242

The large subunit of phage {lambda} terminase, gpA, the gene product of the phage A gene, interacts with the small subunit, gpNu1, to form functional terminase. Terminase binds to {lambda} DNA at cosB to form a binary complex. The terminase:DNA complex binds a prohead to form a ternary complex. Ternary complex formation involves an interaction of the prohead with gpA. The amino terminus of gpA contains a functional domain for interaction with gpNu1, and the carboxy-terminal 38 amino acids of gpA contain a functional domain for prohead binding. This information about the structure of gpA was obtained through the use of hybrid phages resulting from recombination between {lambda} and the related phage 21. {lambda} and 21 encode terminases that are analogous in structural organization and have ca. 60% sequence identity. In spite of these similarities, {lambda} and 21 terminases differ in specificity for DNA binding, subunit assembly, and prohead binding. A {lambda}-21 hybrid phage produces a terminase in which one of the subunits is chimeric and had recombinant specificities. In the work reported here; a new hybrid, {lambda}-21 hybrid 67, is characterized. {lambda}-21 hybrid 67 is the result of a crossover between {lambda} and 21 in the large subunit genes, such that the DNA from the left chromosome end is from 21, including cosB{phi}21, the 1 gene, and the first 48 codons for the 2 gene. The rest of the hybrid 67 chromosome is {lambda} DNA, including 593 codons of the A gene. The chimeric gp2/A of hybrid 67 binds gp1 to form functional terminase. It is concluded that the specificity determinants for small subunit binding reside in the N-terminal 48 amino acids of gpA and gp2. The primary structures of these 48 amino acid segments of gpA and gp2 are quite different, especially in the first half and the predicted secondary structures are also quite different. The DNA sequence of the prohead binding domain of gp2 was determined. The derived amino acid sequence was found to be quite different from the {lambda} prohead binding domain for the C-terminal 32 amino acids, but the predicted secondary structures were found to be similar. It is suggested that these small functional domains, found at the termini of the gpA polypeptide chain, act as ``straps'' in protein-protein assemblies.

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