Allan Campbell

Cell lines harboring latent viruses are common both in eukaryotes and in prokaryotes. Prokaryotes harboring latent phages are lysogenic, and the latent form of the phage is called a prophage. Phages that can enter such a latent state are called temperate, although some authors erroneously refer to them as lysogenic.

Lysogeny was discovered because some bacterial isolates spontaneously produce small amounts of infectious phage. It was later shown that lysogenic bacteria could arise during laboratory infection.

Lysogeny is heritable within a bacterial lineage. When a lysogenic cell divides, both daughter cells harbor the pro-phage. Very occasionally, a lysogenic cell spontaneously lyses and liberates phage. Lwoff and Gutmann (5) demonstrated this by separating bacterial cells at division by micromanipulation and testing their descendants. Their results eliminated the possibility that lysogeny might be an artifact of reinfection within a culture.

The existence of lysogeny poses three basic questions, which have diverse answers among the various known temperate phages: (i) What is the physical nature of the prophage? (ii) What ensures that the prophage replicates during each division cycle and is segregated to both daughters at cell division? (iii) Since the prophage must contain all the genes needed to carry out a lytic cycle, what restrains it from doing so? The first two questions are very closely connected. The third was a cornerstone of Jacob and Monod's (4) proposal that transcription initiation is a general mechanism for controlling gene expression.

Although all temperate phages must achieve regular inheritance and control of lytic functions, they accomplish this in diverse ways. The purpose here is neither to chronicle all this variety nor to detail the mechanisms of particular phages discussed in other chapters. This chapter attempts rather to outline some frequently used mechanisms, emphasizing their relevance to the general goals for which they were selected.

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