The cos sites in λ and 21 chromosomes contain binding sites that recruit terminase to initiate DNA packaging. The small subunits of terminase, gpNu1 (λ) and gp1 (21), have winged helix-turn-helix DNA binding domains, where the recognition helixes differ in four of nine residues. To initiate packaging, the small subunit binds three R sequences in the cosB subsite. λ and 21 cannot package each other[U+05F3]s DNA, due to recognition helix and R sequence differences. In λ and 21 cosBs, two bp, tri1 and tri2, are conserved in the R sequences yet differ between the phages; they are proposed to play a role in phage-specific packaging by λ and 21. Genetic experiments done with mixed and matched terminase and cosB alleles show packaging specificity depends on favorable contacts and clashes. These interactions indicate that the recognition helixes orient with residues 20 and 24 proximal to tri2 and tri1, respectively.
Sippy, J., Patel, P., Vahanian, N., Sippy, R., & Feiss, M. (2015). Genetics of critical contacts and clashes in the DNA packaging specificities of bacteriophages λ and 21. Virology, 476, 115–123. https://doi.org/10.1016/j.virol.2014.11.028