GENETIC-ANALYSIS OF COSB, THE BINDING-SITE FOR TERMINASE, THE DNA PACKAGING ENZYME OF BACTERIOPHAGE-LAMBDA

cosB, the binding site for terminase, the DNA packaging enzyme of bacteriophage λ, consists of three binding sites (called R3, R2 and R1) for gpNu1, the small subunit of terminase; and I1, a binding site for integration host factor (IHF), the DNA bending protein of Escherichia coli. cosB is located between cosN, the site where terminase introduces staggered nicks to generate cohesive ends, and the Nu1 gene; the order of sites is: cosN-R3-I1-R2-R1-Nu1. A series of λ mutants have been constructed that have single base-pair C-to-T transition mutations in R3, R2 and R1. A single base-pair transition mutation within any one of the gpNu1 binding sites renders λ dependent upon IHF for plaque formation. λ phage with mutations in both R2 and R3 are incapable of plaque formation even in the presence of IHF. Phages that carry DNA insertions between R1 and R2, from 7 to 20 base-pairs long, are also IHP-dependent, demonstrating the requirement for a precise spacing of gpNu1 binding sites within cosB. The IHF-dependent phenotype of a λ mutant carrying a deletion of the R1 sequence indicates that IHF obviates the need for terminase binding to the R1 site. In contrast, a λ mutant deleted for R2 and R1 fails to form plaques on either IHF+ or IHF- cells, indicating terminase binding of R2 is involved in suppression of R mutants by IHF. A fourth R sequence, R4, is situated on the left side of cosN; a phage with a mutant R4 sequence shows a reduced burst size on both an IHF+ and an IHF- host. The inability of the R4- mutant to be suppressed by IHF, plus the fact that R4 does not bind gpNu1, suggests R4 is not part of cosB and may play a role in DNA packaging that is distinct from that of cosB. © 1992.