Control of Integration and Excision

Abstract

INTRODUCTION
Phage λ controls integration and excision as a crucial aspect of regulated development along the lysogenic or lytic pathways (Echols 1980; Herskowitz and Hagen 1980). The lysogenic response to infection requires two coordinated events: establishment of repression and integrative recombination. The lytic response to infection utilizes an alternative switch to a virus-producing late stage of productive growth, and integration and cI-mediated repression do not occur. The lytic pathway after prophage induction requires, in addition, the excision of the viral DNA. In terms of these three aspects of λ development, we can define three states of the integration-excision switch: integration, excision, and no response.

Control of the integration-excision reaction depends on the existence of two pathways for catalysis of site-specific recombination: (1) the forward (insertion) reaction, which requires only Int among phage-specified proteins (Gingery and Echols 1967; Zissler 1967; Gottesman and Yarmolinsky 1968), and (2) the reverse (excision) reaction, which requires the phage-coded Int and Xis proteins (Fig. 1) (Guarneros and Echols 1970; Kaiser and Masuda 1970). Thus, differential synthesis of Int with respect to Xis should favor insertion. In this paper we summarize information about mechanisms for control of Int and Xis production that explain quite well the three stages of the integration-excision switch noted above. We also consider some additional regulatory features of the reaction. In closing, we comment on properties of this biological system that may be pertinent to other creatures.

GENERAL FEATURES OF DIRECTIONAL CONTROL
Phage λ uses differential expression of the tightly linked, partially overlapping...