Figure 1. Implication of recombination in the replication of Lambda phage.

Packaging of Lambda chromosomes requires concatemeric DNA. The induction of the lytic cycle leads to a number of rounds of theta replication (circle-to-circle). Concatemeric DNA can be formed directly from these newly replicated chromosomes by homologous recombination using the Red pathway, which requires the recombinase Redβ and the exonuclease Redα, or the host RecBCD pathway of recombination specifically enhanced by Chi sites. Concatemers can also be produced by rolling-circle (sigma) replication if the host RecBCD exonuclease is inhibited (e.g. by Gam encoded in Lambda). Concatemers are cleaved by the phage-encoded terminase at their cos sites (represented in orange) as they are packaged into the capsid. Lambda encoded sequences are indicated in red, the host encoded genes in blue. ori indicates the origin of replication.