Lambda Immediate Early and Delayed Early Genes Are Needed for Both Lysogeny and the Lytic Cycle

KEY CONCEPTS
- Lambda has two immediate early genes, N and cro, which are transcribed by host RNA polymerase.
- The product of the N gene, an antiterminator, is required to express the delayed early genes.
- Three of the delayed early gene products are regulators.
- Lysogeny requires the delayed early genes cII–cIII.
- The lytic cycle requires the immediate early gene cro and the delayed early gene Q.
One of the most intricate cascade circuits is provided by phage lambda. Actually, the cascade for lytic development itself is straightforward, with two regulators controlling the successive stages of development. The circuit for the lytic cycle, though, is interlocked with the circuit for establishing lysogeny, as illustrated in FIGURE 1.

FIGURE 1.The lambda lytic cascade is interlocked with the circuitry for lysogeny.
When lambda DNA enters a new host cell, the lytic and lysogenic pathways start off the same way. Both require expression of the immediate early and delayed early genes, but then they diverge: Lytic development follows if the late genes are expressed, and lysogeny ensues if synthesis of a gene regulator called the lambda repressor is established by turning on its gene, the cI gene. Lambda has only two immediate early genes, transcribed independently by host RNA polymerase:
- The N gene encodes an antitermination factor whose action at nut (N utilization) sites allows transcription to proceed into the delayed early genes . The N gene is required for both the lytic and lysogenic pathways.
- The cro gene encodes a repressor that prevents expression of the c1 gene encoding the lambda repressor (essentially derepressing the late genes, a necessary action if the lytic cycle is to proceed). It also turns off expression of the immediate early genes (which are not needed later in the lytic cycle). The lambda repressor is the major regulator required for lysogenic development.
The delayed early genes, turned on by the product of the N gene, include two replication genes (needed for lytic infection), seven recombination genes (some involved in recombination during lytic infection, two genes necessary to integrate lambda DNA into the bacterial chromosome for lysogeny), and three regulator genes. These regulator genes have opposing functions:
- The cII–cIII pair of regulator genes is needed to establish the synthesis of the lambda repressor for the lysogenic pathway.
- The Q regulator gene codes for an antitermination factor that allows host RNA polymerase to transcribe the late genes and is necessary for the lytic cycle.
Thus, the delayed early genes serve two masters: Some are needed for the phage to enter lysogeny, and the others are concerned with controlling the order of the lytic cycle. At this point, lambda is keeping open the option to choose either pathway.