DNA Packaging Specificity in the λ‐Like Phages: Gifsy‐1

ABSTRACT DNA viruses recognize viral DNA and package it into virions. Specific recognition is needed to distinguish viral DNA from host cell DNA. The λ‐like Escherichia coli phages are interesting and good models to examine genome packaging by large DNA viruses. Gifsy‐1 is a λ‐like Salmonella phage. Gifsy‐1's DNA packaging specificity was compared with those of closely related phages λ, 21, and N15. In vivo packaging studies showed that a Gifsy‐1‐specific phage packaged λ DNA at ca. 50% efficiency and λ packages Gifsy‐1‐specific DNA at ~ 30% efficiency. The results indicate that Gifsy‐1 and λ share the same DNA packaging specificity. N15 is also shown to package Gifsy‐1 DNA. Phage 21 fails to package λ, N15, and Gifsy‐1‐specific DNAs; the efficiencies are 0.01%, 0.01%, and 1%, respectively. A known incompatibility between the 21 helix‐turn‐helix motif and cosB λ is proposed to account for the inability of 21 to package Gifsy‐1 DNA. A model is proposed to explain the 100‐fold difference in packaging efficiency between λ and Gifsy‐1‐specific DNAs by phage 21. Database sequences of enteric prophages indicate that phages with Gifsy‐1's DNA packaging determinants are confined to Salmonella species. Similarly, prophages with λ DNA packaging specificity are rarely found in Salmonella . It is proposed that λ and Gifsy‐1 have diverged from a common ancestor phage, and that the differences may reflect adaptation of their packaging systems to host cell differences.