DNA Packaging by λ-Like Bacteriophages: Mutations Broadening the Packaging Specificity of Terminase, the λ-Packaging Enzyme

The DNA-packaging specificities of phages λ and 21 depend on the specific DNA interactions of the small terminase subunits, which have support helix-turn-recognition helix-wing DNA-binding motifs. λ-Terminase with the recognition helix of 21 preferentially packages 21 DNA. This chimeric terminase9s ability to package λDNA is reduced ∼20-fold. Phage λ with the chimeric terminase is unable to form plaques, but pseudorevertants are readily obtained. Some pseudorevertants have trans -acting suppressors that change codons of the recognition helix. Some of these codons appear to remove an unfavorable base-pair contact; others appear to create a novel nonspecific DNA contact. Helper-packaging experiments show that these mutant terminases have lost the ability to discriminate between λ and 21 during DNA packaging. Two cis -acting suppressors affect cosB , the small subunit9s DNA-binding site. Each changes a cosB λ -specific base pair to a cosB 21 -specific base pair. These cosB suppressors cause enhanced DNA packaging by 21-specific terminase and reduce packaging by λ-terminase. Both the cognate support helix and turn are required for strong packaging discrimination. The wing does not contribute to cosB specificity. Evolution of packaging specificity is discussed, including a model in which λ- and 21-packaging specificities diverged from a common ancestor phage with broad packaging specificity.