The
study of individual operons, such as the lac
operon, has revealed many important details about the mechanisms
of gene regulation. A greater challenge is to understand how
many operons are regulated coordinately. The paradigm is bacteriophage
lambda. Although lambda has only 7 operons, they control two
completely different modes of bacteriophage growth inside the
E. coli host
cell.
[PI.1]
Among the different types of control of gene expression that the phage uses are:
- negative regulation
- positive regulation
- antitermination
- antisense RNA control
- control of RNA degradation
Bacteriophage lambda was discovered by Joshua and Esther Lederberg. They were using UV light to mutagenize strains of E. coli when they found that one of the strains was, in fact, a lysogen.
Bacteriophage lambda has two different life cycles. It can infect its host, E. coli, replicate and synthesize new phage, then lysing and killing the host as the phage burst from the cell. Or, the phage can infect the cell and enter a dormant phase within the cell in which the phage is present but its genes are generally not being expressed. Then under certain circumstances, the phage will leave dormancy and resume lytic growth. Bacterial cells that contain such dormant phage are lysogens -- i.e. they carry the potential to be lysed. The bacteriophage has two modes of growth: lytic and lysogenic.
[P1.2]
The existence and meaning of lysogeny had been controversial ever since Felix d'Hérelle discovered bacteriophage. It was difficult to know whether the sudden appearance of phage in bacterial cultures was due to some contaminant in the cultures or due to some intrinsic but unknown part of the bacteria themselves. Tom Brock gives a nice description of the state of confusion, its resolution, and the importance of lysogeny at the start of Chapter 7 in his book "The Emergence of Bacterial Genetics":
| Lysogeny, the hereditary ability to produce phage, occupies a unique position at the junction of genetics and virology. Lysogeny has played an important role in the formulation of ideas about phage, as well as about bacterial genetics. For many years, lysogeny remained a mystery, even a controversy, and an objective and dispassionate view was not possible. The discoverer of phage, Felix d'Herelle, refused to believe in the existence of a phage carrier state, whereas his key opponent, Jules Bordet, insisted taht at least some viruses lacked virulent characteristics. Later Max Delbrück, concentrating his research on virulent phage, rejected the idea of lysogeny, despite the convincing evidence provided by F.M. Burnet and Eugène and Elisabeth Wollman iin the 1930s. It was only gradually, over a 30-year period, that the nature of the lysogenic state and the relation of virulent to temperate phage became understood, mainly through work after World War II by André Lwoff, Elie Wollman, and François Jacob. The clarification of the nature of lysogeny led ultimately to the development of a strong link between the two major camps, the virological/biophysical, under Delbrück, and the genetic/biochemical, under Lwoff and Monod. The implications of research on lysogeny for gene regulation, cancer, and animal virology are widespread and extremely important. |
| From The Emergence of Bacterial Genetics by Thomas D. Brock (1990), Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY. |
Bacteriophage lambda has a dsDNA genome of 48,502 bp. In the capsid, the chromosome is a linear dsDNA molecule with 12 nt ssDNA cohesive termini. The capsid consists of a HEAD (made up of the B, C, Nu3, D & E gene products) and a TAIL (made up of the products of the J and H genes).
[P1.1]
The bacteriophage attaches to the maltose receptor (product of the E. coli lamB gene) on the surface of the host cell and injects its DNA chromosome. The linear dsDNA circularizes as a result of annealing between the complementary 3' overhanging cos sites.
[P3.2]
Subsequent events depend on whether lytic or lysogenic growth occurs.
If lytic growth occurs, DNA replication then occurs followed by synthesis of the capsid proteins to package the newly-replicated phage. Lambda has two modes of DNA replication:
[7.14]
THETA-form replication
In this form of replication, the product of the O gene binds to four 19 bp palindromes at the origin of replication. Subsequently, the product of the P gene, along with the host cell DnaB protein binds to the origin to establish a replisome. Replication then proceeds using the host cell replication protein machinery in both a bidirectional (75% of the time) and a unidirectional (25% of the time) manner. The O gene product is the phage counterpart of the host cell's DnaA protein; the P gene product if the phage counterpart of the host cell's DnaC protein.
ROLLING CIRCLE replication
Late replication of bacteriophage lambda is designed to produce many copies of the genome in a form suitable for packaging into new phage capsids. How the phage switches from one form of replication to another is not known. The O and P gene products are probably required and the same origin of replication is probably used.
Rolling circle replication generates long multimeric concatemers of the bacteriophage chromosome. Unit-length genomes are cleaved by the TERMINASE enzyme, which is coded by the Nu1 and A genes.
If lysogenic growth occurs, the bacteriophage recombines into the host bacterial chromosome. It does not replicate autonomously -- but it will be replicated each time the host chromosome is replicated.
[P3.4]
