Lambdoid phages with abundant Chi recombination hotspots reflect diverse viral strategies for recombination-dependent growth.

Many phages encode recombination-mediating enzymes, but characterization of their roles in phage lifecycles is limited, and their impact on phage replication is controversial. To address these issues, we have searched for phages whose growth is impacted by the major recombination-promoting helicase-nuclease of , the RecBCD enzyme. Although no phages inhibited by RecBCD are identified, growth of a newly isolated phage, named LLS, is enhanced by RecBCD. LLS's genome sequence reveals it is related to bacteriophage λ but encodes no recombination-promoting (Rec) proteins or associated RecBCD inhibitor. However, it contains an unexpectedly high number of Chi sites, activators of RecBCD-dependent recombination. Through analysis of 325 genomes of phages related to λ (lambdoid phages), we have found 71 other phage genomes that encode no Rec proteins but mostly possess large numbers of Chi sites. Conversely, phages encoding Rec proteins and a RecBCD inhibitor (collectively a Rec module) mostly lack Chi sites. Lambdoid phages of both diverse enteric bacteria and a pseudomonad have these properties. For this study, we thoroughly analyze the Rec modules of 246 lambdoid phage genomes. These analyses reveal a remarkable heterogeneity of Rec module protein types, both in sequence and in function, and allow us to identify phages that do not contain Rec modules. We conclude that phages lacking their own recombination systems have compensated by becoming enriched in Chi sites, enabling them to use the host's RecBCD to fulfill the requirement for recombination to efficiently replicate. This study highlights the importance of recombination for phage survival and the diversity of strategies to achieve it.