Lysogenic Cycle

When Lambda enters host cell, there cro gene product and cI gene product (repressor) race to fill the three binding sites at the left operator (O_L) and right operator (O_R).   If cI wins the race, it preferentially binds first to O_R1  and O_L1, then O_R2  and O_L2.  This blocks the promoter region for the cro gene, so cro is turned off and the virus becomes lysogenic.  Eventually there is enough cI that it binds to the O_R3  and O_L3 sites, turning off the production of additional cI protein.

When UV light breaks down the cI protein product, both cro and cI are turned on, and the race begins again.  If cI wins, lysogeny occurs.  If cro wins, the lytic cycle follows.

Lytic Cycle

The Cro gene product preferentially binds first to O_R3  and O_L3, then O_R2  and O_L2.  This blocks the promoter region for the cI gene, so the cI repressor is turned off and the virus becomes lytic.  Eventually there is enough cro that it binds to the O_R1  and O_L1 sites, turning off the production of additional cro and N protein.

Transcription of the early genes (N and cro) occurs, but is terminated at t_L1 and t_R1.  Once N protein is made, it binds to t_L1 and t_R1, allowing transcription to occur beyond these two points.  The delayed early genes (cII, O, P and R on the right operon and cIII through att on the left operon) are then turned on.

There is an additional termination site at the 3’ end of the Q gene (qut).  Transcription of the delayed early genes ends here.  When the Q protein is made, it binds to qut and allows transcription of the late genes on the late operon (S through J).

Note that the early genes (cro and N) are involved in turning on the lytic cycle and the delayed early genes (cIII through att and cII through Q) are involved in replicating the lambda DNA.  The late genes are involved in making the protein coats that enclose the DNA, as well as genes for breaking the cell open (lysis genes).

Once the lambda infects the next cell, the race between lysogeny and the lytic cycle begins again.