ABSTRACT
Mature particles of bacteriophage lambda are composed of about equal amounts of protein and DNA. Each phage contains one double-stranded DNA molecule encapsulated in an icosahedral head, which is about 50 nm (0.05 μ) in diameter, and from this projects a flexible tubular tail about 150 nm (0.15 μ) long terminating in a fiber.31 Lambda is an obligatory parasite of Escherichia coli. Growth begins when a phage attaches to the host by the tip of its tail and injects its DNA molecule. Lambda is a temperate phage and, thus, can multiply in E. coli by one of three ways. 1) In productive growth, the injected DNA molecule directs the synthesis of numerous gene products; these promote replication of the phage DNA, synthesis of the phage heads and tails, packaging of the DNA into mature phage particles, and eventual lysis of the cell. At 37°C it takes about 40 minutes to complete a lytic cycle, and about 100 infective progeny phage are produced. 2) Lambda DNA can also persist in the host cell as a prophage and replicate passively as an integral part of the bacterial genome. In this case the injected DNA must first direct the synthesis of gene products that promote its insertion into the genome of the host. Next, it promptly turns on the synthesis of the repressor, which blocks transcription of those genes responsible for autonomous lambda DNA replication and most other phage functions. Any fragment of lambda DNA can exist in the form of a defective prophage. 3) Under special conditions, the lambda genome, or its fragment containing the replication genes (autoregulated by the tof product54), can persist in the “carrier” host as a non-integrated plasmid.
