Little lambda, who made thee?

Abstract

The study of the bacteriophage lambda has been critical to the discipline of molecular biology. It was the source of key discoveries in the mechanisms of, among other processes, gene regulation, recombination, and transcription initiation and termination. We trace here the events surrounding these findings and draw on the recollections of the participants. We show how a particular atmosphere of interactions among creative scientists yielded spectacular insights into how living things work.

FIG. 1.

Model of prophage insertion according to Campbell (31), modified to show sequences specific to prophage and bacterial attachment sites. The four attachment sites, shown as zigzag lines, are colored differently in the top part of the figure to indicate that they differ in sequence and function (see the text). The phage chromosome is shown as a solid line, and a segment of the bacterial chromosome is shown as a dashed line. The points of genetic exchange between attP and attB are indicated (×). The positions of phage genes A, J, N, and R and bacterial genes gal and bio are included for orientation. Normal prophage excision occurs by recombination between attL and attR. Formation of a λ gal transducing phage chromosome is illustrated in the bottom part of the figure.

FIG. 2.

Regulation of int expression by RNase III cleavage of sib. Two segments of the λ chromosome with relevant genes and sites are indicated by the two horizontal lines. Transcripts that initiate at P_L are antiterminated following binding of N protein to the nutL site (see the text). These transcripts are cleaved at sib by RNase III (indicated by the scissors symbol), and upstream RNA, which includes the Int-coding region, is destabilized by exonucleolytic digestion in a 3′-to-5′ direction (indicated by the “Pacman” symbol). If sufficient CII protein is made, it activates transcription at P_Int, a promoter whose start site is within xis. These transcripts terminate at the T_Int terminator (indicated by the hairpin symbol), do not contain a complete sib, are not cleaved by RNase III, and are relatively resistant to exonucleolytic digestion.

FIG. 3.

Top. Ter action excises a recombinant prophage from a double lysogen (146). The drawing shows a segment of the bacterial chromosome containing two differently marked λ prophages: λ A⁺ R⁻ and λ A⁻ R⁺. The attachment sites are indicated by zigzag lines, and the cos sites are indicated by the circle-arrowhead symbol. Heteroimmune superinfection of this dilysogen excises and packages the recombinant expected from Ter cleavage of the two cos sites (see the text). (Bottom) Packaging of λ docL DNA from a single lysogen (125, 185). Induction of a lysogen carrying a single prophage that is unable to excise from the bacterial chromosome leads to abundant production of a noninfectious particle carrying the left cohesive end and prophage and bacterial DNA located to the right of cos.

FIG. 4.

Model for cos cleavage and DNA packaging (adapted from reference with permission of the publisher). (a) Circular DNA of a cos duplication mutant replicates to produce a concatemer, which is a substrate for Ter cleavage. cos is indicated by the circle-arrowhead symbol (○>). Cleavage separates the left cohesive end (>) from the right (○). The two copies of cos are labeled a and b, and the approximate sizes, in kilobase pairs, of the λ chromosome segments between them are shown. The types of chromosome and their observed frequencies in the progeny phage population were determined by electron microscopy of DNA heteroduplexes and are shown at the right. (b) According to the model, the first Ter cleavage occurs with equal probability at any of the cos copies on a concatemer. Cleavage is linked to DNA packaging, and packaging proceeds rightwards from the first cleaved cos. Several λ chromosomes are cleaved and packaged in a continuous sequence from a typical concatemer. If these assumptions are correct and production of both the tiny and the large ab fragments is rare, aa chromosomes should be more frequent than ba chromosomes, as was observed. This is because only aa chromosomes can follow cleavage of an a cos site in a packaging series on a single concatemer. By contrast, the assumption of leftward packaging predicts an excess of bb over aa chromosomes.

FIG. 5.

Patterns of early transcription (adapted from reference with permission of the publisher). The top line is a genetic map of λ (not to scale) showing functional groups of genes. The two lines marked imm21 and imm434 show the extents of the regions of nonhomology between λ imm21, λ imm434, and λ. The arrows at the bottom indicate the extent and intensity of transcription after infection with λ N⁻ or N⁺. Other symbols are explained in the text.