Proteins Kil, γ, β, and Exo

Proteins Kil, γ, β, and Exo

Leftward transcription beyond CIII leads to the accumulation of the Kil, γ, β, and Exo proteins in addition to others whose functions are notknown but that are nonessential. Kil stops cell division. The γ protein shuts off the host recombination pathway, and the Exo and Î² proteins open a new pathway for genetic recombination.

Figure 14.8 The rolling circle replicative forms are sensitive to RecBCD, butγwill inactive RecBCD, thus sparing the replicative forms.

Turning off the host recombination pathway is essential to the phage. As mentioned above, lambda DNA is first replicated bidirectionally from the ori region. Later in its growth cycle, however, lambda’s replication shifts to a rolling circle mode. This generates the oligomers of lambda DNA that are obligatory for encapsidation. Few, if any, rolling circles survive in the presence of active host RecBCD enzyme. Apparently this enzyme binds at the free end of DNA in a rolling circle and moves toward the circle. To prevent the cell from inactivating the rolling circles, lambda inactivates RecBCD with a protein of its own, γ (Fig. 14.8). Lambda still has need for recombination, and it therefore potentiates another pathway for genetic recombination by synthesizing the Exo and β proteins.