4. The lambda lytic cascade relies on antitermination

11.4 The lambda lytic cascade relies on antitermination

Figure 11.8 The lambda lytic cascade is interlocked with the circuitry for lysogeny.

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. But the circuit for the lytic cycle is interlocked with the circuit for establishing lysogeny, as summarized in Figure 11.8 (for review see Ptashne, 1992).

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; lysogeny ensues if synthesis of the repressor is established.

Lambda has only two immediate early genes, transcribed independently by host RNA polymerase:

• N codes for an antitermination factor whose action at the nut sites allows transcription to proceed into the delayed early genes. We discussed the mechanisms involved in antitermination in 9 Transcription.
  
• cro has dual functions: it prevents synthesis of the repressor (a necessary action if the lytic cycle is to proceed); and it turns off expression of the immediate early genes (which are not needed later in the lytic cycle).

The delayed early genes include two replication genes (needed for lytic infection), seven recombination genes (some involved in recombination during lytic infection, two necessary to integrate lambda DNA into the bacterial chromosome for lysogeny), and three regulators. The regulators have opposing functions:

• The cII-cIII pair of regulators is needed to start up the synthesis of repressor.
  
• The Q regulator is an antitermination factor that allows host RNA polymerase to proceed into the late genes.

So the delayed early genes serve two masters: some are needed for the phage to enter lysogeny, the others are concerned with controlling the order of the lytic cycle.

Figure 11.9 The lambda map shows clustering of related functions. The genome is 48,514 bp.

To disentangle the two pathways, first consider just the lytic cycle. Figure 11.9 gives the map of lambda phage DNA. A group of genes concerned with regulation is surrounded by genes needed for recombination and replication. The genes coding for structural components of the phage are clustered. All of the genes necessary for the lytic cycle are expressed in polycistronic transcripts from three promoters.

Figure 11.10 Phage lambda has two early transcription units; in the "leftward" unit, the "upper" strand is transcribed toward the left; in the "rightward" unit, the "lower" strand is transcribed toward the right. Promoters are indicated by the shaded red or blue arrowheads. Terminators are indicated by the shaded green boxes. Genes N and cro are the immediate early functions, and are separated from the delayed early genes by the terminators. Synthesis of N protein allows RNA polymerase to pass the terminators t_L1 to the left and t_R1 to the right.
Multiple figure

Figure 11.10 shows that there are two immediate early genes, N and cro, which are transcribed by host RNA polymerase. N is transcribed toward the left, and cro toward the right. Each transcript is terminated at the end of the gene. pN is the regulator that allows transcription to continue into the delayed early genes. It is an antitermination factor that suppresses use of the terminators t_L and t_R. (Its mechanism is discussed in 9 Transcription.) In the presence of pN, transcription continues to the left of N into the recombination genes, and to the right of cro into the replication genes.

Figure 11.11 Lambda DNA circularizes during infection, so that the late gene cluster is intact in one transcription unit.
Multiple figure

The map in Figure 11.9 gives the organization of the lambda DNA as it exists in the phage particle. But shortly after infection, the ends of the DNA join to form a circle. Figure 11.11 shows the true state of lambda DNA during infection. The late genes are welded into a single group, containing the lysis genes S-R from the right end of the linear DNA, and the head and tail genes A-J from the left end.

The late genes are expressed as a single transcription unit, starting from a promoter P_R′ that lies between Q and S. The late promoter is used constitutively. However, in the absence of the product of gene Q (which is the last gene in the rightward delayed early unit), late transcription terminates at a site t_R3. The transcript resulting from this termination event is 194 bases long; it is known as 6S RNA. When pQ becomes available, it suppresses termination at t_R3 and the 6S RNA is extended, with the result that the late genes are expressed.

Late gene transcription does not seem to terminate at any specific point, but continues through all the late genes into the region beyond. A similar event happens with the leftward delayed early transcription, which continues past the recombination functions. Transcription in each direction is probably terminated before the polymerases could crash into each other.

Reviews

Ptashne, M. (1992). A Genetic Switch. Cell Press and Blackwell Scientific, Cambridge.