PubMed1995
Terminase, the bacteriophage lambda DNA packaging protein, is a heteromultimer of two subunits, gpNu1 and gpA, the products of genes Nu1 and A, resp. Phage 21 is a lambdoid phage that produces a terminase similar to that of lambda terminase, the subunits of 21 terminase, gp1 and gp2, have the same domain structures of their lambda analog, gpNu1 and gpA, respectively. The lambda and 21 terminases have different DNA binding and prohead binding specificities. When the C-terminal 32 amino residues of gpA replace the C-terminal 32 residues of gp2, the resulting chimeric terminase specifically uses lambda proheads, indicating that the C-terminal 32 residues of gpA are a specificity domain for prohead binding. A second chimeric terminase, in which the C-terminal six residues of gpA are replaced by the C-terminal six residues of gp2, is unable to utilize lambda proheads, and a lambda phage producing this terminase, lambda Are636, is unable to form plaques. In the present work, a pseudorevertant of lambda Are636 was isolated that contained a mutation Bms8, affecting the prohead. The B gene encodes the portal protein of lambda proheads, which forms the special vertex that is thought to serve as (1) the site of DNA entry into the prohead during packaging, (2) the site for DNA exit during DNA injection, and (3) the site of tail attachment during virion assembly. Bms8 is predicted to change residue 331 of gpB from proline to serine. Burst size measurements and in vitro DNA packaging experiments demonstrated allele-specific interactions between the Are636 terminase and Bms8 proheads. That is, wild-type terminase interacted more efficiently with wild-type proheads than with Bms8 proheads, and Are636 terminase interacted with Bms8 proheads more efficiently than with wild-type proheads. Prohead binding by lambda terminase is stimulated by an assembly catalyst, gpFI. In vitro packaging extracts lacking gpFI were used under conditions in which packaging was gpFI-independent. In the absence of gpFI, Are636 terminase interacted most efficiently with Bms8 proheads, and wild-type terminase interacted most efficiently with wild-type proheads. The allele-specific interactions in the absence of gpFI indicate that the Are636 and Bms8 mutations affect direct interactions between terminase and the portal protein, rather than acting indirectly by altering the interactions of terminase and gpB and gpFI.