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. 2005 Mar 22;102(12):4465-9.
doi: 10.1073/pnas.0409839102. Epub 2005 Feb 23.

On the role of Cro in lambda prophage induction

Sine L Svenningsen  1 , Nina Costantino, Donald L Court, Sankar Adhya
Affiliations

On the role of Cro in lambda prophage induction

Sine L Svenningsen et al. Proc Natl Acad Sci U S A. .

Abstract

The lysogenic state of bacteriophage lambda is exceptionally stable yet the prophage is readily induced in response to DNA damage. This delicate epigenetic switch is believed to be regulated by two proteins; the lysogenic maintenance promoting protein CI and the early lytic protein Cro. First, we confirm, in the native configuration, the previous observation that the DNA loop mediated by oligomerization of CI bound to two distinct operator regions (O(L) and O(R)), increases repression of the early lytic promoters and is important for stable maintenance of lysogeny. Second, we show that the presence of the cro gene might be unimportant for the lysogenic to lytic switch during induction of the lambda prophage. We revisit the idea that Cro's primary role in induction is instead to mediate weak repression of the early lytic promoters.

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Figures

Fig. 1.
Fig. 1.
Genetic map of the immunity region of phage λ showing DNA looping and the various reporter gene fusions. (A) The λ immunity region. (B) The CI dimers bound to OL1/OL2 and OR1/OR2 can octamerize to create a DNA loop between the operators. (C) To create our reporter strains we placed part of the λ immunity region between the luc and lacZ reporter genes at the lac locus. NC414 and NC415 carry a rexA::luc and a cII::lacZ fusion. (D) NC416 and NC417 carry an N::luc and a cII::lacZ fusion.
Fig. 2.
Fig. 2.
Morphology of NC414–417 after growth at 30°C on MacConkey lactose agar (20). The colonies were grown for 42 h after restreaking from single colonies.
Fig. 3.
Fig. 3.
β-Galactosidase activity as a function of growth temperature. (A) The activity of β-galactosidase at growth temperatures in the range between 22°C and 42°C. (B) A magnification of the left part of A, showing the activity of β-galactosidase at low temperatures. Open squares, NC414 (ΔOL cro+); open circles, NC415 (ΔOL cro–); filled squares, NC416 (formula image cro+); filled circles, NC417 (formula image cro–). Assays were performed after 18 h of growth at the indicated temperatures. The absolute values reported here are different from those presented in Fig. 4 and Table 2, in which cases log phase cells were used. The average activity of two to four independent cultures are shown in Miller units.
Fig. 4.
Fig. 4.
β-Galactosidase activity after thermal induction. (A) The β-galactosidase activities of NC414, NC415, NC416, and NC417 at various time points after shift of growth temperature from 30°C to 42°C normalized to A600 at the time of the temperature shift. Open squares, NC414 (ΔO–L cro+); open circles, NC415 (ΔOL cro–); filled squares, NC416 (formula image cro+); filled circles, NC417 (formula image cro–). The β-galactosidase activities of cultures maintained at 30°C has been subtracted from all values. (B) A magnification of the left part A. The average activity of two to four independent cultures is shown.
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